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Life Science 




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This material is licensed under the Creative Commons Attribution-Share Alike license 
(http://creativecommons.0rg/licenses/by-sa/3.o/). 

Print date:2009-06-12 08:19 

CK12 psn: 08aebdc6f417d7371eb07c346396f42c 



Authors 

Jean Brainard, Niamh Gray- Wilson, Jessica Harwood, 
Corliss Karasov, Doris Kraus, and Jane Willan. 



Supported by CK-12 Foundation 



IV 



Contents 



1. Studying Life 1 

2. Introduction to Living Organisms 31 

3. Cells and Their Structures 55 

4. Cell Functions 69 

5. Cell Division, Reproduction, and DNA 89 

6. Genetics 111 

7. Evolution 133 

8. Prokaryotes 173 

9. Protists and Fungi 189 

10. Plants 207 

11. Introduction to Invertebrates 239 

12. Other Invertebrates 257 

13. Fishes, Amphibians, and Reptiles 293 

14. Birds and Mammals 325 

15. Behavior of Animals 353 

16. Skin, Bones, and Muscles 381 

17. Food and the Digestive System 417 

18. Cardiovascular System 447 

19. Respiratory and Excretory Systems 481 

20. Controlling the Body 505 

21. Diseases and the Body's Defenses 545 

22. Reproductive Systems and Life Stages 575 

23. From Populations to the Biosphere 607 

24. Ecosystem Dynamics 643 

25. Environmental Problems 663 



VI 



1. Studying Life 



The Nature of Science 

Lesson Objectives 

• Understand that science is a system based on evidence, testing, and reasoning. 

• Describe what the life sciences are and some of the many life science specialties. 

• Describe the scientific method and why it is important. 

• Define the words "fact," "theory," and "hypothesis." 

• Describe some of the tools of life science. 

• Know that scientists are required to follow strict guidelines. 

Check Your Understanding 

• What do you expect to learn from this class? 

Introduction 

• Know that science is a way of knowing about the physical world, based on observable evidence, testing, 
predictions, and reasoning. 

• In science, theories and knowledge are constantly tested and questioned. When new information conflicts 
with existing explanations, scientists modify their explanations to be consistent with all evidence. 

• Understand that principles of philosophy and religion usually cannot be tested scientifically, because 
they are not based on observable evidence. 

• Identify what the life sciences are and some of the many specialties and know the difference between 
scientific theory and fact. Why is modern science producing many more improvements in our lives than 
it did a hundred years ago? Is there anything that science cannot explain? How can we "think like scien- 
tists?" 

• Know how to ask questions about the world around you and how to answer those questions based on 
your understanding of evidence, testing and reasoning. 

• Continually question and test the accuracy of your knowledge and assumptions. 
Goals of Science 

Science, religion, mythology, and magic share the goal of knowing about and explaining the world, such as 
the physical world, but their approaches are vastly different. The difference between them is their approach 
to "knowing." The vastness of the living, physical world includes all organisms. As humans, some of the 
things we want to know and understand are what makes us healthy, what makes us sick, and how we can 
protect ourselves from floods, famine and drought. 




Figure 1: Escherichia coli bacteria. 

(Source: http://en.wikipedia.0rg/wiki/lmage:EscherichiaC0li_NIAID.jpg, License: Public Domain) 




Figure 2: A male lion. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Li0n_waiting_in_Nambia.jpg, License: Creative Commons 

Attribution 2.0) 




Figure 3: A Humpback whale. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Humpback_stellwagen_edit.jpg, License: GNU Free 

Documentation) 



Throughout history, humans have looked for ways to understand and explain the physical world. Try to 
imagine what humans thought about themselves and the world around them 1 ,000 years ago, or 5,000 years 
ago, or more. If you were born then, how would you have explained why the sun moved across the sky, 
then disappeared? How would you explain why your body changes as you grow, or birth and death? What 
explanation would you have for lightening, thunder, and storms? 




Figure 4: The anatomy lesson of Dr. Nicolaes Tulp. 

(Source: http://en.wikipedia.org/wiki/lmage: The_Anatomy_Lesson.jpg, License: Public Domain) 




Figure 5: In 1 847, a doctor, Ignaz Semmelweis, demonstrated that when he washed his hand before deliv- 
ering babies fewer women died from infection. Before this, doctors held untested beliefs about the causes 
of disease, such as a person's behavior, or the air they breathed. 

(Source: Wikipedia, Artist: Robert A. Thorn, License: Public Domain) 

Throughout time, different cultures have created hundreds of different myths and stories and even gods to 
explain what they saw. Ancient Greeks explained that lightening was a show of their god Zeus' anger. 
Scandinavians claimed that their god of thunder, Thor, was responsible for the rumbling and bolts of lightning. 
Without any formal science, many cultures have also blamed diseases, such as epilepsy, on evil spirits and 
other imaginary entities. For example, there is evidence that many different cultures drilled holes in the skulls 



of patients who had seizures or other maladies, thinking that they were releasing evil spirits. 

Science as a Way of Knowing 

During yours and your parents' lifetimes, advances in medicine, technology, and other fields have progressed 
faster than any other time in history. This explosion of advances in our lives is largely due to human use of 
modern science as a way of understanding. Today's scientists are trained to base their comprehension of 
the world on evidence and reasoning rather than belief and assumptions. 

Modern science is: 

• A way of understanding about the physical world, based on observable evidence, reasoning, and repeated 
testing. 

• A body of knowledge that is based on observable evidence, experimentation, reasoning, and repeated 
testing. 

As we learn more, new information occasionally conflicts with our current understanding. When this happens 
scientific explanations are revised. However, science cannot scrutinize what is good versus what is bad 
(morality), because these are values, ideas that lack measurable evidence. Science is not used to examine 
philosophy or supernatural entities, such as the existence or nonexistence of a god. However, science can 
be used to examine the effects of these experiences. 

The most important message from this chapter is that science is not only a way of knowing it is also a way 
of thinking and reasoning. Scientists try to look at the world objectively - without bias or making assumptions. 
How? Scientists learn to be skeptical, to question the accuracy our ideas. They learn to base their under- 
standing of the physical world on evidence, reasoning and repeated testing of ideas. 

To Think Like a Scientist 

To think like a scientist, you need to be skeptical about and question your assumptions, including what often 
seems like common sense. Questioning ideas can often lead to surprising results. For example, if you ask 
people whether it's easier to keep a plastic cutting board clean or a wooden one clean, most people will 
think that the plastic board is easier to keep clean and has fewer germs. 




Figure 6: Which is safer, a plastic or wood cutting board? 

Why do most people believe that plastic is safer? Probably because we assume that it is easier to wash 
germs off plastic than off wood. This assumption is promoted by the makers of plastic cutting boards and it 
sounds reasonable. After all, wood stains and looks unhygienic; plastic cutting boards come out of the 
dishwasher shiny and clean looking. But is plastic actually better? 

When scientists tested this idea, the answer turned out to be no. The researchers treated used cutting boards 
with different kinds of germs and then washed the boards. They found, much to their surprise, that gouged 



and sliced wooden cutting boards had far fewer germs than gouged and sliced plastic boards. The researchers 
discovered that germs that cause food poisoning, such as E. coli and Salmonella, are absorbed into the 
wood and seemed to vanish. On plastic, the germs sit on the surface in cuts in the plastic where they are 
difficult to clean out but can contaminate food. Furthermore, in a different study of food poisoning, people 
who used wooden cutting boards were less than half as likely to get sick as people using plastic ones. 

"Common sense" may seem to have all the answers, but science is all about following the evidence. So 
what is good evidence? Evidence is information that can be used to confirm or refute an idea or to explain 
something. Both scientists and lawyers use evidence to support an idea or to show that an idea is probably 
wrong. Scientific evidence has certain features, which may be different from legal evidence. 

Evidence is: 

1 . a direct, physical observation of a thing, a group of things, or of a process over time. 

2. usually something measurable or "quantifiable." 

3. the result of something. 

For example, a book falling to the ground is evidence in support of the theory of gravity. A bear skeleton in 
the woods would be supporting evidence for the presence of bears. 

What Are the Life Sciences? 

The life sciences are the study of living organisms and how they interact with each other and their environment. 
These include all the biological sciences. Life sciences deal with every aspect of living organisms. The life 
sciences are so complex that most scientists focus on just one or two subspecialties. 

For example, some focus on the relationship between living organisms, depicted in a phylogenetic "Tree of 
Life" Figure 7. 



Phylogenetic Tree of Life 



Bacteria 



Archaea 



Eucaryota 



Spirochetes 

Proteobacteria 
Cyano bacteria 

Planctomyces 

Bacteroides 
Cytophaga 

Thermotoga 

Aquifex 



Green 

Filamentous 

bacteria 



Ent amoebae 



Slime 
molds 




Animals 
Fungi 



Plants 
Ciliates 



Flagellates 

Trichomonads 

Microsporidia 

Diplomonads 



Figure 7: The Phylogenetic Tree of Life shows the relationship between living organisms. Humans and 
other mammals (eukaryotes) appear on the right side of the tree. The base of the tree represents the ancestor 
of all living organisms. 



(Source: http://en.wikipedia.0rg/wiki/lmage:Phyl0genetic_tree.svg, License: Public Domain) 
Each of the following life science subspecialties focuses on one type of organism: 



Subspecialty 


Studies 


Botany 


plants 


Zoology 


animals 


Marine biology 


organisms living in and around oceans, and seas 


Fresh water biology 


organisms living in and around freshwater lakes, streams, rivers, ponds, etc. 


Microbiology 


microorganisms 


Bacteriology 


bacteria 


Virology 


viruses 


Entomology 


insects 


Taxonomy 


the classification or organisms 



Other fields of life sciences examine the structure, function, growth, development and/or evolution of living 
things: 



Life Science 


What it Examines 


Cell biology 


cells and their structures 


Anatomy 


the structures of animals 


Morphology 


the form and structure of living organisms 


Physiology 


the physical and chemical functions of tissues and organs 


Immunology 


the mechanisms inside organisms that protect them from disease 
and infection 


Neuroscience 


the nervous system 


Developmental biology and embryology 


the growth and development of plants and animals 


Genetics 


the genetic make up of all living organisms (heredity) 


Biochemistry 


the chemistry of living organisms 


Molecular biology 


biology at the molecular level 


Epidemiology 


how diseases arise and spread 




Figure 8: Epidemiologists study how diseases spread. The above map shows where humans contracted 
West Nile Virus between 2000 and 2006. It is believed the virus entered the United States in New York City 
in 1 999. Notice how rapidly the virus spread across the U.S. 



(Source: http://en.wikipedia.0rg/wiki/lmage:WNVUSAMap.png, License: GNU Free Documentation) 
Other fields of biology examine the distribution and interactions between organisms and their 
environments: 



Life Science 


What it Examines 


Ecology 


how various organisms interact with their environments 


Biogeography 


the distribution of living organisms 


Population biol- 
ogy 


the biodiversity, evolution, and environmental biology of populations of organisms 




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Figure 9: Alexander von Humboldt mapped the distribution of plants across landscapes and recorded a 
variety of physical conditions such as pressure and temperature. Today, biogeographers study the diversity 
and distribution of organisms across Earth. 

(Source: http://en.wikipedia.org/wiki/lmage: Humboldt1805-chimborazo.jpg, License: Public Domain) 
Scientific Theories 

Science theories are produced through repeated studies, usually performed and confirmed by many individ- 
uals. Scientific theories are well established and tested explanations of observations. These theories produce 
a body of knowledge about the physical world that is collected and tested through the scientific method 
(discussed in the Scientific Method lesson). 

The word "theory" has a very different meaning in daily life than it does in science. When someone at school 
says, "I have a theory," they sometimes just mean a hunch or a guess. This everyday meaning for "theory" 
can confuse people when well-tested and widely accepted scientific theories are discussed by nonscientists. 
For example, the theory of evolution is a well-established scientific theory that some people incorrectly say 
is just a hunch. 

A scientific theory is based on evidence and testing that supports the explanation. Scientific theories are so 
well studied and tested that it is extremely unlikely that new data will discredit them. The idea that matter is 
made up of atoms, evolution, and gravity are all scientific theories about how the world works that scientists 
accept as fundamental principles of basic science. However, any theory may be altered or revised to make 
it consistent with new evidence. 

Two Important Life Science Theories 

In the many life sciences, there are possibly hundreds or thousands of theories. Yet there are at least two 
fundamental theories, which provide a foundation for modern biology. They are: 



1. The Cell Theory 



2. The Theory of Evolution 

The Cell Theory States That: 

• All organisms are composed of cells. 

• Cells are the basic units of structure and function in an organism. 

• Cells only come from preexisting cells; life comes from life. 

The development of the microscope in the mid 1 600s made it possible to come up with this theory. 




Figure 10: The two types of cells, eukaryotic (left) and prokaryotic (right). 
(Source: http://upload.wikimedia.Org/wikipedia/commons/8/83/Celltypes.svg) 




Figure 11: A mouse cell viewed through a microscope. 

(Source: http://commons.wikimedia.Org/wiki/File:Cellsize.jpg, License: GNU Free Documentation) 

The Theory of Evolution States That: 

In biology, evolution is the process of change in the inherited traits of a population of organisms over time. 
Natural selection is the process where organisms that are better suited to the environment are more likely 
to survive and reproduce than others that are less suited to the environment. This theory basically states 
that better suited organisms live longer and have an easier time reproducing, passing on their traits that 
made them better suited to their environment. The theory of evolution by natural selection is often called 
the "great unifier" of biology, because it applies to every field of biology. It also explains the tremendous di- 
versity and distribution of organisms across Earth. All living organisms on Earth are descended from common 
ancestors. 




Figure 12: Evolution explains the millions of varieties of organisms on Earth. 
(Source: commons.wikimedia.org/wiki/lmage:Animal_diver) 

Lesson Summary 

• Science is a way of understanding (knowing) about the physical world that is based on evidence, reason- 
ing, and testing predictions. 

• A body of knowledge that has been thoroughly tested can still undergo further testing, and revisions as 
new evidence and 

questioning are raised. 

• Science differs from other ways of knowing, because it is entirely based on observable evidence and its 
explanations are 

constantly questioned and tested. 

• Science produces theories and general knowledge that allow us to better understand the world and to 
apply this knowledge to 

solve problems. 
Review Questions 

1 . How is modern science different from other ways of knowing? 

2. Explain why science cannot be used to examine whether someone is good or bad? 



3. How is the scientific meaning of the word "theory" different from its use in day-to- day conversation? 

4. What do all fields of life science have in common? 

5. What are the three characteristics of evidence? 

6. What is the goal of science? 

7. What would you study if you were a biogeographer? 

Further Reading I Supplemental Links 

• Moore, John, A., Science as a Way of Knowing: The Foundations of Modem Biology. Harvard University 
Press, 1993. 

• Trefil, James, The Nature of Science, An A-Z Guide to the Laws and Principles Guiding the Universe. 
Houghton Mifflin, Boston, 2003. 

• Darwin, Charles, Origin of the Species. Random House 1988. 

• Cromer, Alan, Uncommon Sense: The Heretical Nature of Science, Oxford University Press. 1 993. 

• The Nature of Science the Prentice Hall Science Series 1993. 

• American Association for the Advancement of Science. Science for All Americans. 1 993. 

• Johnson, Rebecca, Genetics (Great Ideas of Science), Twenty-First Century Books, 2006. 

• Hedrick, Philip, W., Genetics of Populations (Biological Science (Jones and Bartlett)) Jones and Brothers 
Publishers, 2005. 

• Charles Darwin: And the Evolution Revolution (Oxford Portraits in Science) by Rebecca Stefoff. 
Oxford Press. New York, 1996. 

• Fleisher, Paul, Evolution (Great Ideas of Science). TwentyFirst Century Books, 2006.. 

• Trefil, James, The Nature of Science, An A-Z Guide to the Laws and Principles Guiding the Universe. 
Houghton Mifflin, 2003. 

• Darwin, Charles, Origin of the Species. Random House, 1988. 

• http://www.project2061.org/publications/bsl/online/index.php?chapter=1 
http://www.project2061.org/publications/bsl/online/index. php?chapter=1] 

• http://evolution.berkeley.edu/evosite/nature/index.shtml 
http://evolution.berkeley.edu/evosite/nature/index.shtml] 

• http://en.wikipedia.org/ http://en.wikipedia.org/] 
Vocabulary 

anecdotal evidence A description of an event that is used to make a point. 

biogeography The study of the distribution of living organisms. 

ecology The study of the interactions of organisms with each other and with their environment. 



10 



evidence Something that gives us grounds for knowing of the existence or presence of some- 

thing else. 

life science The study of living organisms and how they interact with each other and their envi- 

ronment. 

population biology The study of the biodiversity, evolution, and environmental biology of populations of 
organisms. 

science A way of knowing about the physical world, based on observable evidence, testing 

predictions, and reasoning. 

science theories Well established and tested explanations of observations; produced through repeated 
studies, usually performed and confirmed by many individuals. 

Review Answers 

1. Science is based on observable evidence and testing rather than belief or mythology. 

2. There is no physical evidence of "good" or "bad" that science can test. Ethics, and belief systems do 
not produce any testable evidence. 

3. In day-to-day use, the word "theory" can mean anything from a hunch to a well-thought out idea; In 
science, a theory is an idea that is well-supported by evidence and testing. 

4. All fields of the life sciences examine features of living (or previously living) organisms. 

5. Scientific evidence must: 

Be empirical (based on observation, not theory). Be measurable. Have an effect on something. 

6. To understand and explain the physical world around us. 

7. The distribution and diversity of organisms around the world. 

Points to Consider 

• Next we are going to discuss the scientific method. You may have heard someone say that you can ruin 
your eyes if you sit too close to the television set. 

• Describe how "thinking like a scientist" could help you figure out if this common sense idea is true or 
false. 

The Scientific Method 

Lesson Objectives 

• Consider how the scientific method is one of the most important reasons for how modern science is 
advancing more rapidly than in the past. 

• Describe the scientific method as a process. 

• Explain why the scientific method allows scientists and others to examine the physical world more 



11 



objectively than other ways of knowing. 

• Describe the steps involved in the scientific method. 

Check Your Understanding 

• What is science? 

• What is a scientific theory? 

Introduction 

The scientific method is an inquiry process used to investigate the physical world using observable evidence 
and testing. This method allows scientists to "conduct" science in a uniform process. This process allows 
the information collected to be reproduced by other scientists, and most importantly, this process allows the 
information to be accepted and trusted. 

Observations, Data, Hypotheses, and Experiments 

Imagine that you are scientist who wants to know something like, "Why do whales migrate?" or "Why do 
some people get more colds than others do?" Two hundred years ago you could have come up with theories 
without necessarily thoroughly testing your ideas. But there were many exceptional scientists who made 
outstanding contributions. Figure 1 shows Michael Faraday in his laboratory in the Royal Institution in England 
during the 1800s. Michael Faraday is best known for his contributions to chemistry, and he probably used 
some form of the scientific method to answer his questions. 




Figure 1 : Michael Faraday in his laboratory at the Royal Institution during the mid 1 800s. 

(Source: http://en.wikipedia.org/wiki/lmage: M_Faraday_Lab_H_Moore.jpg, License: Public Domain) 

As a modern scientist today, you would use the scientific method, collecting evidence to test your hypothesis 
and answer your questions. The scientific method presents a general idea of how science is conducted; it 
is not a strict pattern for doing research. Scientists use many different variations of the scientific method to 
meet their specific needs. Almost all versions of the scientific method include the following steps, though 
not always in the same order: 



12 



1 . Make observations 

2. Identify a question you would like to answer about the observation 

3. Research: find out what is already known about your observation 

4. Form a hypothesis 

5. Test the hypothesis 

6. Analyze your results 

7. Communicate your results 

A hypothesis is a proposed explanation that allows you to make predictions about what ought to happen 
if the hypothesis is true. If the predictions are accurate, that provides support for the hypothesis. If the pre- 
dictions are incorrect, that suggests the hypothesis is wrong. 

1. Make Observations 

Observe something in which you are interested. Here is an example of a real observation made by students 
in Minnesota (Figure 2). Imagine that you are one of the students who discovered this strange frog. 




Figure 2: A frog with an extra leg. 

(Source: http://upload.wikimedia.Org/wikipedia/en/a/af/Deformed_Frog.gif, License: Public Domain) 

Imagine that you are on a field trip to look at pond life. While collecting water samples, you notice a frog 
with five legs instead of four. As you start to look around, you discover that many of the frogs have extra 
limbs, extra eyes or no eyes. One frog even has limbs coming out of its mouth. You look at the water and 
the plants around the pond to see if there is anything else that is obviously unusual like a source of pollution. 

2. Identify a Question That is Based on Your Observations 

The next step is to ask a question about these frogs. For example, you may ask why so many frogs are 
deformed. You may wonder if there is something in their environment causing these defects. You could ask 
if deformities are caused by such materials as water pollution, pesticides, or something in the soil nearby. 

Yet, you do not even know if this large number of deformities is "normal" for frogs. What if many of the frogs 
found in ponds and lakes all over the world have similar deformities? Before you look for causes, you need 
to find out if the number and kind of deformities is unusual. So besides finding out why the frogs are deformed, 
you should also ask: 

"Is the percentage of deformed frogs in pond A (your pond) greater than the percentage of deformed frogs 
in other places?" 



13 




Figure 3: A pond with frogs. 

(Source: http://en.wikipedia.Org/wiki/lmage:Pond02.jpg, License: GNU Free Documentation) 

3. Research Existing Knowledge About the Topic 

No matter what you observe, you need to find out what is already known about your topic. For example, is 
anyone else doing research on deformed frogs? If yes, what did they find out? Do you think that you should 
repeat their research to see if it can be duplicated? During your research, you might learn something that 
convinces you to alter your question. 

4. Construct a Hypothesis 

A hypothesis is a proposed explanation of an observation. For example, you might hypothesize that a certain 
pesticide is causing extra legs. If that's true, then you can predict that the water in a pond of healthy non 
deformed frogs will have lower levels of that pesticide. That's a prediction you can test by measuring pesticide 
levels in two sets of ponds, those with deformed frogs and those with nothing but healthy frogs. A hypothesis 
is an explanation that allows you to predict what results you will get in an experiment or survey. 

The next step is to state the hypothesis formally. A hypothesis must be "testable." 

Example: 

After reading about what other scientists have learned about frog deformities, you predict what you will find 
in your research. You construct a hypothesis that will help you answer your first question. 

Any hypothesis needs to be written in a way that it can: 

a. Be tested using evidence. 

b. Be falsified (found false/wrong). 

c. Provide measurable results. 

d. Provide yes or no answers. 

For example, the following hypothesis can be tested and provides yes or no answers: 

"The percentage of deformed frogs in five ponds that are heavily polluted with a specific chemical X is higher 
than the percentage of deformed frogs in five ponds without chemical X." 

5. Test Your Hypothesis 

The next step is to count the healthy and deformed frogs and measure the amount of chemical X in all the 
ponds. This study will test the hypothesis. The hypothesis will be either true or false. 



14 



An example of a hypothesis that is not testable would be: "The frogs are deformed because someone cast 
a magic spell on them." You cannot make any predictions based on the deformity being caused by magic, 
so there is no way to test a magic hypothesis or to measure any results of magic. There is no way to prove 
that it is not magic, so that hypothesis is untestable and therefore not interesting to a scientist. 

6. Analyze Data and Draw a Conclusion 

If a hypothesis and experiment are well designed, the experiment will produce measurable results that you 
can collect and analyze. The analysis should tell you if the hypothesis is true or false. 

Example: 

Your results show that pesticide levels in the two sets of ponds are statistically different, but the number of 
deformed frogs is almost the same when you average all the ponds together. Your results demonstrate that 
your hypothesis is either false or the situation is more complicated than you thought. This gives you new 
information that will help you decide what to do next. Even if the results supported your hypothesis, you 
would probably ask a new question to try to better understand what is happening to the frogs and why. When 
you are satisfied that you have accurate information, you share your results with others. 

You will probably revise your hypothesis and design additional experiments along the way. 

7. Communicate Results 

Scientists communicate their findings in a variety of ways. For example, they may discuss their results with 
colleagues, talk to small groups of scientists, give talks at large scientific meetings, and write articles for 
scientific journals. Their findings may also be communicated to journalists. 

Example: 

You eventually decide that you have strong results to share about frog deformities. You write an article and 
give talks about your research. Your results could contribute towards solutions. 

Drawing Conclusions and Communicating Results 

If a hypothesis and experiment are well designed, the results will indicate whether your hypothesis is true 
or false. If a hypothesis is supported by the results of a study, scientists will often continuing testing the hy- 
pothesis in new ways to learn more. 

If a hypothesis is false, the results may be used to construct and test a new hypothesis. The next step is to 
analyze your results and to communicate them to other scientists. Scientific articles include the questions, 
methods and the conclusions from their research. Other scientists may try to repeat the experiments or 
change them. Scientists spend much time sharing and discussing their ideas with each other. Different sci- 
entists have different kinds of expertise they can use to help each other. When many scientists have inde- 
pendently come to the same conclusions, a scientific theory is developed. A scientific theory is a well-estab- 
lished explanation of an observation. In is generally accepted among the scientific community. Scientific 
theories are discussed in The Nature of Science Lesson. 

Basic and Applied Science 

Science can be "basic" or "applied." The goal of basic science is to understand how things work - whether 
it's why things fall on the floor or the structure of cells. Basic science is the source of most scientific theory 
and new knowledge. Applied science is using scientific discoveries to solve practical problems or to create 
new technologies. 

Even though basic research is not intended to solve problems directly, basic research always provides the 
knowledge that applied scientists need to solve problems. For example, medicine and all that is known about 
how to treat patients is applied science based on basic research (Figure 4). 



15 




Figure 4: A healthy newborn being examined by a doctor. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Newborn_Examination_1967.jpg, License: GNU Free 
Documentation) 

Lesson Summary 

• The scientific method is an inquiry process used to investigate the physical world using observable 
evidence and testing. 

• A hypothesis is a proposed explanation of an observation; it is used to test an idea. 

• A theory is a well-established explanation of an observation. A hypothesis must be written in a way that 
can be tested, is 

falsifiable (to be able to prove that something is false), is measurable, and will help answer the original 
question. 

Review Questions 

1 . How is a hypothesis different from a theory? 

2. What does a hypothesis need to include? 

3. What does "falsifiable" mean? 

4. List the steps of the Scientific Method? 

5. What is basic research? 

6. What is applied research? 

7. What does a scientist do if their research results conflict with previous theories or popular knowledge? 

8. Is it OK for scientists to change their ideas? 

Further Reading I Supplemental Links 

• William Souder, A Plague of Frogs: The Horrifying True Story Hyperion Press, 2000. 



16 



• http://en.wikipedia.org/ 
Vocabulary 

applied science The application of science to practical problems. 

basic science Research whose goal is just to find out how the world works, not to solve an urgent 
problem. Basic research is the source of most new scientific information and nearly all 
new theories. 

falsifiable Testable. If a hypothesis generates predictions that can be shown to be true or false 

by experiment or observation, the hypothesis is "falsifiable" or "testable." 

hypothesis A proposed explanation for something that is testable. 

predict To say what will happen in a given situation. A scientific prediction is different from an 

everyday prediction, like predicting the weather before it happens. A scientific prediction 
is related to a specific hypothesis. 

scientific method A careful way of asking and answering questions to learn about the physical world that 
is based on reason and observable evidence. 

scientific theory A well-established set of explanations that explain a large amount of scientific informa- 
tion. 

Review Answers 

1. A hypothesis is a proposed explanation of an observation. It is used to test an idea. A theory is a well- 
established explanation of an observation; it has already undergone testing and well supported by evidence. 

2. A hypothesis must be written in a way that is testable, falsifiable, and measurable. 

3. Falsifiable means that if something is false it's possible to show that it is false. 

4. The Scientific Method Steps include: 

a. Make observations. 

b. Identify a question about the observation that you would like to explore. 

c. Research what is already known about your observation. 

d. Construct a hypothesis. 

e. Test your hypothesis. 

f. Analyze your results. 

g. Communicate your results. 

5. Basic research is research done to advance knowledge, not designed to meet a need. 

6. Applied research is designed to meet a need. Applied research usually uses knowledge from basic 
research. 

7. Theories and knowledge are altered to be consistent with evidence from the research. 

8. Absolutely yes! Scientists are trained to base their understanding of the world on evidence and 



17 



testing, even if it differs from their previous ideas. 

Points to Consider 

• Next we consider the tools of the scientist. 

• How do you think scientific "tools" can help a scientist? 

• What do you think is one of the more common tools of the life scientist? 

Tools of Science 

Lesson Objectives 

• Describe the growing number of tools available to investigate different features of the physical world. 

• Describe how microscopes have allowed humans to view increasingly small tissues and organisms that 
were never visible before. 

Check Your Understanding 

• What is the scientific method? 

• What is an experiment? 

Using Microscopes 

Microscopes, tools that you may get to use in your class, are some of the most important tools in biology 
Figure 1. Before microscopes were invented in 1595, the smallest things you could see on yourself were 
the tiny lines in your skin. The magnifying glass, a simple glass lens, was developed about 1200 years ago. 
Atypical magnifying glass may have doubled the size of an image. But microscopes allowed people to see 
objects as small as individual cells and even large bacteria. Microscopes let people see that all organisms 
are made of cells. Without microscopes, some of the most important discoveries in science would have 
been impossible. 




18 



Figure 1 : Basic light microscopes opened up a new world to curious people. 1 , ocular lens or eyepiece; 2, 
objective turret; 3, objective lenses; 4, coarse adjustment knob; 5, fine adjustment knob; 6, object holder or 
stage; 7, mirror or light (illuminator); 8, diaphragm and condenser. 

(Source: http://en.wikipedia.Org/wiki/lmage:Optical_microscope_nikon_alphaphot.jpg, License: Wikimedia 
Commons) 

Microscopes are used to look at things that are too small to be seen by the unaided eye. Microscopy is 
a technology for studying small objects using microscopes. A microscope that magnifies something two to 
ten times (indicated by 2X or 1 0X on the side of the lens) may be enough to dissect a plant or look closely 
at an insect. Using even more powerful microscopes, scientists can magnify objects to two million times 
their real size. 

Some of the very best early optical microscopes were made four hundred years ago by Antoine van 
Leeuwenhoek, a man who taught himself to make his own microscopes (Figure 2, 3). When he looked at a 
sample of scum from his own teeth, Leeuwenhoek discovered bacteria. In rainwater, he saw tiny protozoa. 
Imagine his excitement when he looked through the microscope and saw this lively microscopic world, van 
Leeuwenhoek discovered the first one-celled organisms (protists), the first bacteria, and the first sperm. 
Robert Hooke, an English natural scientist of the same period of history, used a microscope to see and 
name the first "cells" (Figure 4), which he discovered in plants. 




Figure 2: Antoine van Leeuwenhoek, a Dutch cloth merchant with a passion for microscopy. 
(Source: http://en.wikipedia.Org/wiki/lmage:Antoni_van_Leeuwenhoek.png, License: Public Domain) 



19 




Figure 3: Drawing of microscopes owned by Antoine van Leeuwenhoek. Bacteria were discovered in 1 683 
when Antoine Van Leeuwenhoek used a microscope he built to look at the plaque on his own teeth. 

(Source: http://en.wikipedia.Org/wiki/lmage:Van_Leeuwenhoek%27s_microscopes_by_Henry_Baker.jpg, 
License: Wikimedia Commons) 




Figure 4: Robert Hooke's early microscope. 

(Source: http://en.wikipedia.Org/wiki/lmage:Hooke-microscope.png, License: Public Domain) 

Some modern microscopes use light, as Hooke's and van Leeuwenhoek's did, but others may use electron 
beams or sound waves. 

Researchers now use four kinds of microscopes: 



20 



1 . Light microscopes allow biologists to see small details of biological specimens. Most of the microscopes 
used in schools and laboratories are light microscopes. Light microscopes use refractive lenses, typically 
made of glass or plastic, to focus light either into the eye, a camera, or some other light detector. The most 
powerful light microscopes can magnify images up to 2,000 times. Light microscopes are not as powerful 
as other higher tech microscopes but they are much cheaper and anyone can own one and see many 
amazing things. 

2. Transmission electron microscopes (TEM) focus a beam of electrons through an object and can 
magnify an image up to two million times (Figure 5) with a very clear image ("high resolution"). 

3. Scanning electron microscopes (SEM) allow scientists to map the surfaces of extremely small objects. 
These microscopes slide a beam of electrons across the surface of specimen, producing detailed maps of 
the shapes of objects. 

4. Scanning acoustic microscopes use sound waves to scan a specimen. These microscopes are useful 
in biology and medical research. 




Figure 5: A scanning electron microscope. 

(Source: http://en.wikipedia.Org/wiki/lmage:Loupe-binoculaire-p1030881.jpg, License: CC-BY-SA France) 

Other Life Science Tools 

What other kinds of tools and instruments would you expect to find in a biologist's laboratory or field station? 
Other than computers and lab notebooks, biologists use very different instruments and tools for the wide 
range of life science specialties. For example, a medical research laboratory and a marine biology field 
station might not use any of the same tools (Figures 6-8). 



21 




Figure 6: A radiotelemetry device used to track the movement of seals in the wild. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/6/69/Phoca_vitulina_Telemetry.jpg This is a file 
from the Wikimedia Commons) 




Figure 7: A thermocycler used for molecular biological and genetic studies. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Pcr_machine.jpg, License: Public Domain) 



22 




Figure 8: A laboratory fume hood. This laboratory hood sucks dangerous fumes out of a lab and allows re- 
searchers to work with dangerous chemicals without breathing them. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/0/02/Fume_hood.jpg, License: Public Domain) 

Using Maps and Other Models 

You use models for many purposes. A volcano model, is not the same as a volcano, but it is useful for 
thinking about real volcanoes. We use street maps to represent where streets are in relation to each other. 
A model of planets may show the relationship between the positions of planets in space. Biologists use 
many different kinds of models to simulate real events and processes. Models are often useful to explain 
observations and to make scientific predictions. 

Some models are used to show the relationship between different variables. For example, the model in figure 
9 says that when there are few coyotes, there are lots of rabbits (left side of the graph) and when there are 
only a few rabbits, there are lots of coyotes (right side of the graph). You could make a prediction, based 
on this model, that removing all the coyotes from this system would result in an increase in rabbits. That's 
a prediction that can be tested. 



Model of reliationship between coyotes and rabbits 




# of coyotes 



Figure 9: This graph shows a model of a relationship between a population of coyotes (the predators) and 
a population of rabbit, which the coyotes are known to eat (the prey). 



23 



(Created by: Talia Karasov, License: CC-BY-SA) 

Lesson Summary 

From the time that the first microscope was built, over four hundred years ago, microscopes have been 
used to make major discoveries. 

Life science is a vast field; different kinds of research usually require very different tools. 

Basic research produces knowledge and theories; applied research uses knowledge and theories from 
basic 

esearch to develop solutions to practical problems. 

Scientists use maps and models to understand how features of real events or processes work. 

Review Questions 

1 . What did van Leeuwuhoek discover when he looked at plaque from his own teeth under the microscope? 

2. What does the symbol 1 0X on the side of a microscope mean? 

3. What is a scientific model? 

4. Look at the predator/prey (coyote/rabbit) model again. What does the model predict would happen to the 
rabbit population if you took away all the coyotes? 

5. How long ago were the first microscopes invented? 

6. What tool would you use to keep track of where a wolf travels? 

7. What is the relationship between basic and applied research? 

Further Reading I Supplemental Links 

• http://en.wikipedia.org/ 

Vocabulary 

applied research Research designed for the purpose of producing results that may be 

applied to real world situations. 

basic research Research to gain new knowledge about the basic processes of life, in- 

cluding how the body works; but the goal is not a commercial application. 

electron microscopes Used to create high magnification (magnified many times) and high 

resolution (very clear) images. 

microscopes A set of lenses used to look at things too small to be seen by the unaided 

eye. 

microscopy All the methods for studying things using microscopes. 

optical (light) microscopes A microscope that focuses light, usually through a glass lens; used by 

biologists to small details of biological specimens. 

scanning acoustic microscopes A microscope that focuses sound waves instead of light. 



24 



scanning electron microscopes A microscope that scans the surfaces of objects with a beam of electrons 

to produce detailed images of the surfaces of tiny things. 

Review Answers 

1 . Bacteria 

2. The lens magnifies the image ten times. 

3. A scientific model is a simplified way of understanding a real situation. A model can be a physical model, 
showing the structure of a molecule or the arrangements of planets around the sun, or a simple mathemat- 
ical relationship expressed as an equation or a graph. 

4. The rabbit population would be large. 

5. 400 years ago 

6. Radiotelemetry 

7. Applied research depends on discoveries from basic research. 

Points to Consider 

• What could be some hazards that biologists may face in the laboratory? 

• What could be risks of doing field research? 

• So what do you think biologists do to protect themselves? 

Safety in Scientific Research 

Lesson Objectives 

• Recognize how the kind of hazards that a scientist faces depends on the kind of research they do. 

• Identify some potential risks associated with scientific research. 

• Identify who and what safety regulations are designed to protect. 

Check Your Understanding 

• What is the scientific method? 

Introduction 

There are some very serious safety risks in scientific research. Research can involve many different kinds 
of risks. Yet, if science were as dangerous as some horror movies make it look, not many people would 
become scientists. Since the life sciences deal with living organisms, some research may have risks not 
found in other fields. Safety practices are needed to work with any potentially hazardous situation, such as: 

• pathogenic (disease-causing) viruses, bacteria or fungi 

• parasites 

• wild animals 

• radioactive materials 



25 



pollutants in air, water, or soil 

toxins 

teratogens 

carcinogens 

radiation 

The kinds of risks that scientists face depend on the kind of research they perform. For example, a bacteri- 
ologist working with bacteria in a laboratory faces different risks than a zoologist studying the behavior of 
lions in Africa. Think back to the deformed frogs discussed earlier, the ones in the pond with extra limbs or 
extra eyes. If there is something in the frogs' environment causing these deformities, could there be a risk 
to a researcher in that environment? A chemical in the pond that could cause such deformities is called a 
"teratogen." Or perhaps a disease is causing the deformities. Infectious agents such as viruses and bacteria 
are called biohazards. Biohazards include any material such as medical waste that could possibly transmit 
an infectious disease. A used hypodermic needle or a vial of bacteria are both biohazards. 




Figure 1: The Biohazard symbol. 

Laboratory Safety 

Most laboratories are safe places to visit. If you plan to work in a scientific laboratory, ask someone to tell 
you about the safety rules they are required to follow. Scientists must follow regulations set by federal, state, 
and private institutions. For example, scientists cannot work with hazardous materials or equipment without: 

• Getting approval to do the specific research. 

• Using safety equipment, such as hoods and fans (figure 2). 

• Demonstrating that the staff are familiar with risks, know how to respond to problems, and can follow 
safety regulations. 

• Accepting laboratory inspections by safety officers at any time. 



26 




Figure 2: An example of a science laboratory workbench. A fume hood is on the left. 
(Source: http://upload.wikimedia.Org/wikipedia/commons/5/5f/Lab_bench.jpg) 




Figure 3: Scientists studying dangerous organisms such as Yersinia pestis, the cause of bubonic plague, 
use special equipment that helps keep the organism from escaping the lab. 

(Source: http://en.wikipedia.org/wiki/Black_Death, License: Public Domain) 
Field Research Safety 

Scientists who work in the outdoors, called "field scientists," are also required to follow safety regulations 
designed to prevent harm to themselves, other humans, to animals, and the environment. 

Scientists are required to follow the same level of safety standards in the field as they do in a laboratory. In 
fact, if scientists work outside the country, they are required to learn about and follow the laws and restrictions 
of the country in which they are doing research. For example, entomologists following monarch butterfly 
migrations between the United States and Mexico would have to follow regulations in both countries. 



27 




Figure 4: A Monarch Butterfly 

(Source: http://en.wikipedia.org/wiki/lrnage:Monarch_Butterfly_Danaus_plexippus_on_Milkweed_Hy- 

brid_2800px.jpg, Photographed By: Derek Ramsey, License: GNU Free Documentation) 

Field scientists are also required to follow laws to protect the environment. Before biologists can study pro- 
tected wildlife or plant species, they must apply for permission to do so, and obtain a research permit, if re- 
quired. 

Lesson Summary 

• Research of any kind may have safety risks. Because biologists study living organisms as diverse as 
bacteria and bears, they deal with risks that other scientists may never encounter. 

• The risks scientists face depend on the kind of research they are doing. 

• Scientists are required by federal, state, and local institutions to follow strict regulations designed to 
protect the safety of themselves, the public, and the environment. 

Review Questions 

1 . What kinds of hazards might be found in biology laboratories, but not physics laboratories? 

2. Who has more freedom to do whatever research they want? Laboratory scientists or field biologists? 

3. What is a biohazard? 

4. What is a research permit? 

5. What are some of the precautions you might take if you were collecting frogs in water you think might be 
polluted? 

6. Name some possible hazards to field biologists. 

7. If a scientist does research in a foreign country, which research laws would the scientist need to 



28 



follow: those of the homeland or the foreign country? 

Further Reading I Supplemental Links 

• Biosafety in Microbiological and Biomedical Laboratories (National Research Council, 1999). 

• Chemical Classification Signs: http://www.howe.k12.ok.us/~jimaskew/nfpa.htm. 
http://www.howe.k12.ok.us/~jimaskew/nfpa.htm. 

• NFPA Chemical Hazard Labels: http://www.atsdr.cdc.gov/NFPA/nfpa_label.html. 
http://www.atsdr.cdc.gov/NFPA/nfpa_label.html. 

• Where to Find MSDS's on the Internet: http://www.ilpi.com/msds/index.html. 
http://www.ilpi.com/msds/index.html. 

• Cornell University MSDS: http://msds.pdc.cornell.edu/msdssrch.asp. 
http://msds.pdc.cornell.edu/msdssrch.asp. 

• MSDS Power Point: http://www.tenet.edu/teks/science/safety/pdf/hazcom/msds.ppt. 
http://www.tenet.edu/teks/science/safety/pdf/hazcom/msds.ppt. 

• http://www.research.northwestern.edu/ors/biosafe/index.htm. 
http://www.research.northwestern.edu/ors/biosafe/index.htm. 

• http://en.wikipedia.org/ http://en.wikipedia.org/. 
Vocabulary 

anecdotal evidence A description of an event that is used to make a point. 

applied research Research designed for the purpose of producing results that may be applied to real 
world situations. 

basic research Research to gain new knowledge about the basic processes of life, including how 

the bodyworks normally; but the goal is not a commercial application. 

biohazard Is any biological material, such as infectious material that poses a potential to human 

health, animal health, or the environment. 

evidence Something used to clearly determine or demonstrate the truth of an assertion. 

falsifiable Confirmable; capable of being tested (verified or falsified) by experiment or observa- 

tion. 

hypothesis A concept that is not yet verified but that if true would explain certain facts or phenom- 

ena. 

pathogen A disease causing agent. 

scientific model Something used to represent feature a real system or item. 

theory An explanation for an event that is based on observation, experimentation, and rea- 

soning. 

Review Answers 

1 . Hazards associated with working with living organisms including bacteria and viruses. The answer may 



29 



include any kind or biohazards 

2. Neither. Both laboratory and field biologists are required to follow all of the regulations 

3. Biological waste that is infectious; something that can be passed on to others 

4. Permission from government or private institutions to conduct specific kinds of research. 

5. You might wear protective gear, wash yourself thoroughly after handling any frogs, and you could keep 
frogs away from any food you eat. Other reasonable ideas are acceptable as well. 

6. Field biologists might need to protect themselves from: 

a. Air, water, or soil pollution 

b. Infectious agents and parasites 

c. Predators and other dangerous animals 

7. Scientists need to follow the scientific laws of the country they are in as well as the laws of the United 
States (if they are affiliated with a US institution and/or have a grant from the US government). 

Points to Consider 

• We are now moving into examining living things. 

• What do you think makes something "alive?" 

• What may be some things a blade of grass, a fly, and you have in common? 



30 



2. Introduction to Living Organisms 



What are Living Things? 

Lesson Objectives 

• List the defining characteristics of living things. 

• List the needs of all living things. 

Check Your Understanding 

• How do life scientists study the natural world? 

• Are scientific theories just a "hunch" or a hypothesis? 

introduction 

How would you define a living thing? In other words, what do mushrooms, daisies, cats, and bacteria have 
in common? All of these are living things, or organisms. It might seem hard to think of similarities among 
such diverse organisms, but there are actually many similarities. The chemical processes inside all organisms 
are the same. For example, all living things encode their genetic information in the same way. And many 
organisms share the same needs, such as the need for energy and materials to build their bodies. Living 
things have so many similarities because all living things have evolved from the same common ancestor 
that lived billions of years ago. 

All living organisms: 

Need energy to carry out life processes 

Are composed of one or more cells (the cell theory) 

Evolve and share an evolutionary history 

Respond to their environment 

Grow, reproduce themselves, and pass on information to their offspring in the form of genes 

Maintain a stable internal environment (homeostasis) 

Insert Composite MS LS Ch2.1 .1 Image Here 

Living Things Maintain Stable Internal Conditions 

All living things have some ability to maintain a stable internal environment. The inside of an organism is 
separate and different from the outside world. Maintaining that separation and difference is known as 
homeostasis. For example, many animals work hard to keep their temperature within a certain range. If 
the animal gets too hot or too cold, it will die. As a result, many animals have evolved behaviors that regulate 
their internal temperature. A lizard may stretch out on a sunny rock to increase its internal temperature, and 
a bird may fluff its feathers to stay warm. 



31 




Figure 2: A bird fluffs his feathers to stay warm (keep from losing energy) and to maintain homeostasis. 

Mammals and birds are homeotherms-meaning they maintain the same temperature most of the time. A 
lizard or an earthworm is a heterotherm, meaning its temperature can change. 

Humans and other mammals may deliberately do things to stay warm or to cool off, like lie down under a 
shady tree. But most mammals maintain a steady temperature primarily through unconscious processes. 
A portion of your unconscious brain regulates your body temperature. If you get too warm, you start to sweat 
and the blood vessels in your skin open up to let the blood flow to the surface of your body. If you are too 
cold, you start to shiver and the blood supply to your skin, hands and feet may be reduced. 

There are many forms of homeostasis besides temperature regulation. For example, when you have a big 
lunch, your body produces the hormone insulin, which helps maintain the right amount of sugar in your 
blood. Meanwhile, your kidneys are hard at work maintaining the right amount of water and salts in your 
blood. Both of these processes happen unconsciously and are part of homeostasis. 

Living Things Grow and Reproduce 

All living things reproduce. Organisms that do not reproduce go extinct, every time. As a result, there are 
no species that do not reproduce. 




Figure 3: Like all living things, cats reproduce themselves and make a new generation of cats. When animals 
and plants reproduce they make tiny undeveloped versions of themselves called embryos, which grow up 
and develop into adults. A kitten is a partly developed cat. 



32 



Reproduction, the process of creating a new organism, is different for different organisms. Many organisms 
reproduce sexually, where an egg and sperm go together to form a new individual. Other organisms can 
reproduce without sex ("asexually"). For example, bacteria can simply split in two, producing two identical 
new cells. But it's not just bacteria that can reproduce without sex. Some lizards can produce clones of 
themselves. In such species, all individuals are female and simply lay their eggs when they are ready to 
reproduce. During all reproduction, the parents pass genetic information to their offspring, a process called 
heredity. Heredity is the passing of genes to the next generation. These genes influence all the traits of an 
organism, including overall body shape, size, whether it has fur or feathers, teeth or a beak, eye color, and 
so on. This genetic information is essential to an organism. In all organisms made of cells, this genetic infor- 
mation comes in the form of deoxyribonucleic acid, or DNA, which we will discuss in lesson 2. (In viruses, 
which are not made of cells, the genetic information is sometimes in the form of RNA, a different nucleic 
acid.) DNA contains the "instructions" for building important molecules inside of cells. 

Living Things are Composed of Cells 

All living things are composed of cells, the tiny units that are the building blocks of life. Cells are the smallest 
possible unit of life that is still considered living. Most cells are so small that they are usually visible only 
through a microscope. Some organisms, like the tiny plankton that live in the ocean, are composed of just 
one cell. Other organisms have many millions of cells that make up different body tissues and organs. On 
the other hand, eggs are some of the biggest cells around, including chicken eggs and ostrich eggs. But 
most cells are tiny. 




Figure 4: Skin cells. All living organisms are made of one or more cells. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Epithelial-cells.jpg, License: GNU-FDL) 

[image:Lsc-0201 -04.jpg|200px] 

Figure 5 



33 




Figure 5: This Paramecium is a one-celled organism. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Paramecium.jpg, License: GNU_FD) 

All cells share at least some structures. But there are thousands of kinds of cells with different structures. 
The cells of plants and mushrooms have a cell wall, while the cells of animals do not. The cells of most or- 
ganisms have a special membrane around the DNA, but bacterial cells do not. Although the cells of different 
organisms are built differently, they all function much the same way. Every cell must get energy from food, 
be able to grow and reproduce, and respond to its environment. 

Living Things Need Resources and Energy 

In order to grow, reproduce, and maintain homeostasis, living things need energy. The work you do each 
day, from walking to writing and thinking, is fueled by energy in your cells. But where does this energy come 
from? 

The source of energy differs for each type of living thing. In your body, the source of energy is the food you 
eat. All animals must eat plants or other animals in order to obtain energy and building materials. Plants 
themselves don't eat; they use the energy of the sun to make their "food" through the process of photosyn- 
thesis (see Cell Functions chapter). Like animals, mushrooms and other fungi obtain energy from other or- 
ganisms. That's why you often see fungi growing on a fallen tree; the rotting tree is their source of energy. 
Although the means of getting energy might be different, all organisms need some source of energy. And 
since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the 
energy of living things ultimately comes from the sun. 




34 



Figure 6: Fungi obtain energy from breaking down dead organisms, such as this rotting log. 

(Source: http://www.flickr.com/photos/pixbybill/935712719/, License: CC-BY-SA) 

Besides obtaining energy from the foods you eat, you also need the chemical building blocks in food to build 
and maintain your body. For example, you get calcium for building bones from eating dairy products or leafy 
greens. Plants obtain nutrients from the soil. Nutrients will be discussed in the next lesson. 

Lesson Summary 

• All living things grow, reproduce, and maintain a stable internal environment. 

• All organisms are made of cells. 

• All living things need energy and resources to survive. 

Review Questions 

1 . Define the word organism. (Beginning) 

2. Give two examples of processes that help organisms achieve homeostasis. (Intermediate) 

3. What are three characteristics of living things? (Intermediate) 

4. What are a few ways organisms can get the energy they require? (Challenging) 

5. What is a cell? (Intermediate) 

Further Reading I Supplemental Links 

• http://publications.nigms.nih.gov/thenewgenetics/thenewgenetics.pdf 

• http://en.wikipedia.org 

Vocabulary 

cell The smallest living unit of life; the smallest unit of structure of living organisms. 

DNA Deoxyribonucleic acid; the heredity material; carries the genetic information of the cell. 

heredity The passing of traits or a tendency to certain traits to the next generation through units of 

inheritance called genes. 

homeostasis Maintaining a stable internal environment despite changes in the environment. 

organism A living thing. 

reproduction The process by which an organism makes a new organism with at least some of its own 
genes. 

Review Answers 

1 . A living thing 2. Filtering the blood to achieve water balance with kidneys; burning energy to stay warm; 
secrete insulin to regulate sugar. 3. Grow, Reproduce, Maintain a stable internal environment, Composed 
of cells. 4. Photosynthesis, eat plants, eat animals, decompose other organisms, absorb food from a host 
(parasite such as a flea, tapeworm, or bacterium). 5. The smallest living units of life. 

Points to Consider 

• DNA is considered the "instructions" for the cell. What do you think this means? 

• What kinds of chemicals do you think are necessary for life? 



35 



Do you expect that the same chemicals can be in non-living and living things? 



Chemicals of Life 

Lesson Objectives 

• Distinguish between an element and a compound. 

• Explain how elements are organized on the periodic table. 

• Explain the function of enzymes. 

• Name the four main classes of organic molecules that are building blocks of life. 

Check Your Understanding 

• What are the main properties of all living things? 

• What is homeostasis? 

Introduction 

Physical science and biology are two different subjects in school, so you might see them as two unrelated 
sciences. However, understanding physical science is essential for understanding biology. Living things are 
subject to the same physical laws of the universe as non-living things. The rules that apply to chemical re- 
actions in a test tube also apply to the chemical reactions that take place inside your body. To understand 
how living things function, we must have a little knowledge of physics and chemistry. This includes knowing 
what elements are and how different molecules come together to form the components of life. 

The Elements 

Rocks, animals, flowers, and even your body, are made up of matter. Matter is anything that takes up space 
and has mass. Matter makes up everything, living and nonliving. 

Matter is composed of a mixture of elements. Elements are substances that cannot be broken down into 
simpler substances with different properties. Even chemical reactions or physical processes, like heating 
or crushing, cannot break it down to release a simpler substance. There are more than 1 00 known elements, 
and 92 occur naturally around us. The others have been made only in the laboratory. 

Elements are made up of identical atoms. An atom is the simplest and smallest particle of matter that still 
retains the chemical properties of the element. Atoms are so tiny that only the most powerful microscopes 
can detect them. Atoms are the building block of all elements, and of all matter. Each element has a different 
type of atom, and is represented with a one or two letter symbol. For example, the symble for oxygen is O 
and the symbol for carbon is C. 

Atoms themselves are composed of even smaller particles, including: the positively charged protons, the 
uncharged neutrons, and the negatively charged electrons. Protons and neutrons are located in the center 
of the atom, or the nucleus, and the electrons move around the nucleus. How many protons and neutrons 
an atom has determines what element it is. For example, Helium (He) always has two protons, while Sodium 
(Na) always has 11. To restate this, all the atoms of a particular element have the exact same number of 
protons, and the number of protons is that element's atomic number. 



36 




Figure 1: An atom of Helium (He) contains two positively charged protons (red), two uncharged neutrons 
(green), and two negatively charged electrons (yellow). 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Atom.svg, License: GNU_FD) 

The Periodic Table 

Each element also has unique properties, such as density, boiling point, and how well it dissolves ("solubility"). 
Density is the mass of the substance per unit of volume. That means that if you take an equal volume of 
different elements, each different sample will weigh a different amount. For example, a liter of the metal 
mercury weighs 13 times as much as a liter of water. The boiling point is the temperature at which an element 
will change from a liquid to a gas. For example, the boiling point of water is 1 00 degrees Celsius. Once you 
heat water to this temperature, you see bubbles form as the water turns into vapor. Each element has a 
different boiling point. Solubility is how well a substance will dissolve in water. You can dissolve more sugar 
in a liter of water than salt, because sugar is more soluble than salt. Density, boiling point, and solubility 
have unchanging values for each element. 

In 1869, Dmitri Mendeleev constructed the periodic table in 1869, organizing all the elements according to 
their atomic number, density, boiling point, solubility, and other values. As mentioned above, each element 
has a one or two letter symbol. For example, H stands for hydrogen and Au for gold. The vertical columns 
in the periodic table are known as groups and elements in groups tend to have very similar properties. The 
table is also divided into rows, known as periods. 

Group 1 (see figure 1 ) contains the highly reactive metals, such as sodium (Na) and lithium (Li). Just a small 
amount of these metals will explode into flames when put into water. Another group are the less-reactive 
metals, such as gold (Au) and platinum (Pt). Since they will not react readily with air and tarnish, these 
metals are highly valued for making jewelry. There are also highly reactive nonmetals, such as chlorine and 
oxygen, and some nonreactive gases, such as helium (He) and neon (N), which you might recognize from 
helium balloons and neon signs. 



37 





H 1 






















1 Me 






3 


4 






1 5 


6 




8 


g 


10 






Li 


Be 








B 


C 


N 





F 


Ne 




11 


12 


13 


14 


15 


is 


17 


18 




Ma 


'y';i | 




1 A 


Si 


P 


s 


CI 


Ar 




19 


20 


21 


22 


23 


24 


25 


26 


27 


28 


29 


3D 


31 


32 


33 


34 


35 


36 




K 


Ca 


Sc 


Ti 


V 


Cr 


Mn 


Fg 


Co 


Ni 


Cu 


Zn 


Ga 


Ge 


As 


Se 


Br 


Kr 




37 


33 


39 


40 


41 


42 


43 


44 


45 


46 


47 


43 


49 


50 


51 


52 


53 


54 




Rb 


Sr 


Y 


Zr 


Nb 


Mo 


Tc 


Ru 


Rh 


Pd 


Ag 


Cd 


In 


5n 


5b 


To 


1 


Xa 




55 


56 




72 


73 


74 


75 


76 


77 


76 


79 


w 


si 


32 


33 


94 


85 


8S 




Cs 


Ba 




Hf 


Ta 


W 


Re 


Os 


lr 


Pt 


Au 


Hg 


TI 


Pb 


Bi 


Po 


At 


Rn 




87 


68 


» 


1(1-1 


105 


106 


107 


106 


109 


110 


111 


112 


113 


114 


114 


115 




118 




Fr 


Ra 






Db 


sg 


Bh 


Hs 


Ml 


Ds 


Rg 


uut> 


Jut 


Uuq 


Uup 


Uuh 


Uus 


Uuo 




■ 1 c 1 


^^^^^^^^ 








Uunl 














57 


56 


59 


60 


61 


62 


63 


64 


65 


66 


67 


68 


69 


70 


71 








La 


Ce 


Pr 


Nd 


Pm 


Sm 


Eu 


Gd 


Tt> 


Dy 


Ho 


Er 


Tm 


Vb 


Lu 




69 


90 


91 


92 


93 


94 


95 


96 


97 


98 


99 


100 


101 


102 


103 






Ac 


Th 


Pa 


U 


Np 


Pu 


Am 


Cm 


Bk 


Cf 


Es 


Fm 


Md 


No 


Lr 




















Alkali metals 


Alkaline earthi metals 1 Lanthamdes 


Actlnldes 


1 




Poor inetals 


Metalloids 1 Nonmetals 


Halogens 


gases ^^^^^^^^^^^^^^^| 





















Figure 2: The periodic table groups the elements based on their properties. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Periodic_Table_Armtuk3.svg, License: GNU-FDL) 

Chemical Reactions 

A molecule is any combination of two or more atoms. The oxygen in the air we breathe is two oxygen atoms 
connected by a chemical bond to form 2 , or molecular oxygen. A carbon dioxide molecule is a combination 

of one carbon atom and two oxygen atoms. Because carbon dioxide includes two different elements it is a 
"compound" as well as a molecule. 

A compound is any combination of two or more elements. A compound usually has very different properties 
from the elements that it contains. Elements and combinations of elements make up all the diverse types 
of matter in the universe. 

The process by which two different elements come together to form a compound is one example of a 
chemical reaction. For example, hydrogen and oxygen together form water. Water has the properties of 
a liquid, not the properties of the gases hydrogen and oxygen. Water is the product, or end result, of the 
chemical reaction while hydrogen and oxygen are the reactants, or "ingredients" necessary for the chemical 
reaction. 

One important chemical reaction in your everyday life is oxidation, or the combination of oxygen and another 
element. Examples of oxidation are burning and rusting. When oxygen combines with gas on your stove 
top, the reaction releases heat that you can use to cook with. (In fact, since fires need oxygen to burn, most 
fire extinguishers are composed of heavier gasses that will displace the oxygen, smothering the fire.) Rust 
is formed when oxygen combines with iron. These are a few examples of chemical reactions. 



38 




Figure 3: Rust is the result of a chemical reaction between iron and oxygen. 

(Source: http://www.flickr.com/photos/hvargas/2114683166/, License: CC- Attribution) 

Enzyme Reactions 

The oxidation reaction occurs readily, but not all reactions move so quickly. Others can take quite a while. 
Since many of the body's necessary chemical reactions would take years to happen on their own, you need 
the help of enzymes. Enzymes, which will be discussed in more detail later, speed up chemical reactions, 
often by bringing the reactants closer together so they can interact more easily. Enzymes attach to, or bind, 
specifically to the reactants. Because enzymes are so specific, you have a different enzyme for every 
chemical reaction in your body. A single cell may contain hundreds or thousands of different enzymes. 

When an enzymes attaches, or binds, to another molecule, that molecule is referred to as the substrate. 
The enzyme is usually much bigger than the substrate. 




Figure 4: The enzyme (green) binds to the substrate (red) to speed up a chemical reaction. 



39 



(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Chim0trypsin-inhib.png, License: Public Commons) 

Organic Compounds 

The chemical components of living things are known as organic compounds, which means they contain 
the element carbon (C). Living things are made up of compounds that are quite large. These large compounds 
molecules, known as macromolecules, are made of smaller molecules. You might recognize some of these 
organic molecules as parts of the food you eat. Through eating food, we obtain the organic molecules we 
need to grow and be healthy. 

Table 1: The Four Main Classes of Organic Molecules 





Proteins 


Carbohydrates 


Lipids 


Nucleic Acids 


Elements 


C,H,0,N,S 


C,H,0 


C,H,0,P 


C,H,0,P,N 


Examples 


Enzymes, mus- 
cle fibers, anti- 
bodies 


Sugar, Starch, 
Glycogen, Cellu- 
lose 


Phospholipids in 
membranes, 
fats, oils, waxes, 
steroids 


DNA, RNA, ATP 


Monomer 

(small building 
block molecule) 


Amino acids 


Sugars 


Often include 
fatty acids 


Nucleotides 



Organic compounds all contain the elements carbon (C)and hydrogen (H). The chain of carbon and hydrogen 
in organic compounds is sometimes called the "backbone" of organic compounds since they make up the 
core center structure. What makes organic compounds different from one another is the functional groups, 
groups of atoms that have unique chemical properties. The addition of a functional group vastly changes 
the properties of the carbon-hydrogen backbone of organic compounds. Each organic compound is therefore 
suited to its unique role in living things. 

Carbohydrates 

Essentially, carbohydrates are sugars or long chains of sugars. An important role of carbohydrates is to 
store energy. Glucose (see sidebar) is a simple sugar molecule with the chemical formula C 6 H 12 6 . Sugar 

is one type of carbohydrate, but carbohydrates also include long chains of connected sugar molecules. 
These chains of sugar molecules can be used to store sugar for later use, such as in the form of starches 
or glycogen. Plants store sugar in long chains called starch, whereas animals store sugar in long chains 
called glycogen. Both storage molecules contain hundreds or thousands of linked glucose molecules. Chains 
of sugar molecules also can be used as structural molecules. For example, the hard skeletons of insects 
and lobsters are made of chitin, a type of carbohydrate. You get the carbohydrates you need for energy 
from eating carbohydrate-rich foods, including fruits and vegetables, as well as grains such as bread, rice, 
or corn. 

The chemical formula C 6 H 12 6 of glucose means that this molecule has 24 

atoms: 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Carbohydrates 
have a general chemical formula consisting of twice as many hydrogen atoms 
as carbon and oxygen atoms. Glucose is a monomer, a single unit that when 
linked together with other monomers forms a long chain known as a polymer. 
Starch is an example of a polymer . 



Proteins 

Proteins have many different functions in living things. Enzymes are a type of protein. Antibodies that protect 
your body from disease are proteins, and your muscles are made of protein. All proteins are made of 
monomers (small building block molecules) called amino acids that line up, like beads on a string, to form 



40 



long chains. There are only 20 common amino acids, which, in different combinations, form thousands of 
unique proteins. After a cell makes a protein chain, the chain folds into a 3-dimensional structure. Each 
folded protein has its own unique shape, a structure that gives the protein its function. 




^X 




Figure 5: Proteins fold into unique 3-dimensional structures, starting with the linear "beads on a string," 
shown at the top, to the complex structure on the bottom. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Pr0teinStructures.gif) 

It's important for you and other animals to eat food with protein because we cannot synthesize some of 
these amino acids ourselves. You can get proteins both from plant sources such as beans and from animal 
sources, like milk or meat. When you eat food with protein, your body breaks the proteins down into individual 
amino acids and uses them to build new proteins. Therefore, you really are what you eat! 

Lipids 

The lipids - the fats, oils, and waxes - are a diverse group of organic compounds. Lipids are not soluble in 
water. (As you probably know, oil and water don't mix.) The most common lipids in your diet are probably 
fats and oils. Fats are solid at room temperature, whereas oils are fluid. Animals use fats for long-term energy 
storage and insulation. Plants use oils for long-term energy storage. When preparing food, we often use 
animal fats, such as lard and butter, or plant oils, such as olive oil or canola oil. 



41 



There are many more type of lipids that are important to life. One of the most important are the phospholipids 
(see the chapter titled Cell Functions) that make up the membranes that surround all cells. Steroids are the 
basis for the hormones like testosterone and estrogen. Waxes are useful lipids for plants and animals since 
they are waterproof. Plants coat their leaves in a waxy covering to prevent water loss, while bees use wax 
to make their honeycombs. 

Nucleic acids 

Nucleic acids are long chains of nucleotides, which are units composed of a sugar, a nitrogen-containing 
base, and a phosphate group. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two main 
nucleic acids. DNA is the molecule that stores our genetic information and RNA is involved in making proteins. 
Nucleotides also make up the high-energy molecule Adenosine Triphosphate (ATP). ATP is the energy 
currency of the cell. Everytime you think a thought or move a muscle, you are using the energy stored in 
ATP. 



CH,OH 




b) 



c) 







HHaiBHil 


iiUiiTti'iuflJii'isi'uriii' 


In 1 


YyyYTyYiYi. 


'TyiYTyxYy 



42 




Figure 6: There are many examples of each of the categories of organic compounds. 

Pictured are: a) a molecule of glucose (a carbohydrate) b) muscle fibers (protein) c) phospholipids in a 
membrane (lipid) d) DNA (nucleic acid) 

(Sources: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Arm_muscles_fr0nt_superficial.png, License: Public 
Domain; http://c0mm0ns.wikimedia.0rg/wiki/lmage:Bilayer_scheme. svg,<nowiki> "License:" Public Domain; 
<nowiki>http://commons.wikimedia.org/wiki/lmage:ADN_animation_%28no_animated%29.png, License: 
Public Domain) 




Figure 7: A healthy diet includes protein, fat, and carbohydrate. 

(Source: http://www.flickr.com/photos/dongkwan/2294951493/, License: Creative Commons) 

Lesson Summary 

Elements are substances that cannot be broken down into simpler substances with different properties. 

Elements have been organized by their properties to form the periodic table. 

Two or more atoms can combine to form a molecule. 

Molecules consisting of more than one element are called compounds. 

Reactants can combine through chemical reactions to form products. 

Enzymes can speed up a chemical reaction. 



43 



• Living things are made of just four classes of macromolecules: proteins, carbohydrates, lipids, and nucleic 
acids. 

Review Questions 

1. What is density? (Intermediate) 

2. What are the 4 main classes of organic compounds? (Beginning) 

3. Would water, with the symbol H 2 0, be considered an element or a compound? (Challenging) 

4. How many types of atoms make up gold? (Intermediate) 

5. Why do you need fats in your diet? (Intermediate) 

6. Sugar is what kind of organic compound? (Beginning) 

7. What is an atom? (Beginning) 

8. What monomers make up proteins? (Intermediate) 

9. Name a few examples of proteins. (Challenging) 

1Q Name a few examples of lipids in organisms. (Challenging) 
11. What are two nucleic acids? (Intermediate) 

Further Reading I Supplemental Links 

• http://ghr.nlm.nih.gov/handbook/howgeneswork/protein 

• http://ghr.nlm.nih.gov/handbook/basics/dna 

• http://publications.nigms.nih.gov/thenewgenetics/chapter1.html 

Vocabulary 

amino acids Monomers that combine to make protein chains. 

atom The simplest and smallest particle of matter that still retains the physical and 

chemical properties of the element; the building block of all matter. 

ATP Adenosine triphosphate, the energy "currency" of the cell. 

carbohydrates Class of organic compound that includes sugar, starch, cellulose and chitin. 

electron A negatively charged particle in the atom, found outside of the nucleus. 

element A substance that cannot break down into a simpler substance with different proper- 

ties. 

enzyme Protein that speeds up a chemical reaction by binding to the reactants (substrates). 

functional groups Groups of atoms that give a compound its unique chemical properties. 

lipids Class of organic compound that includes fats, oils, waxes and phospholipids. 

matter Anything that takes up space and has mass. 

neutrons The non-charged particle of the atom; located in nucleus of the atom. 

nucleic acid Class of organic compound that includes DNA and RNA. 

organic compounds Compounds made up of a carbon backbone and associated with living things. 

phosopholipids Lipid molecule that makes up cell membranes. 

product The end result formed from a chemical reaction. 

protein Class of organic compound consisting of a chain of amino acids; includes enzymes 

and antibodies. 



44 



Proton The positively charged particle of the atom; located in nucleus of the atom. 

Reactants: The raw ingredients in a chemical reaction. 

Waxes A water-proof lipid. 

Review Answers 

1 . The mass of a substance per unit volume. 2. Carbohydrates, lipids, proteins, and nucleic acids. 3. Water 
is a compound composed of the elements hydrogen and oxygen. 4. One, all atoms of an element are the 
same. 5. To make cell membranes, long term energy storage, insulation. 6. Carbohydrate 7. The smallest 
particle of matter that still retains the properties of the element. 8. Amino acids 9. Muscle fibers, enzymes, 
antibodies, etc. 10. Oils, fats, phospholipids in membranes, steroids, wax 11 . DNA and RNA 

Points to Consider 

• Do you expect the genetic information in the DNA of a cow to be the same or different from that in a 
crow? 

• If we are all composed of the same chemicals, how do all organisms look so different? 

• What characteristics would you use to distinguish and classify living things? 



Classification of Living Things 

Lesson Objectives 

• Explain what makes up a scientific name. 

• Explain what defines a species. 

• List the information scientists use to classify organisms. 

• List the three domains of life and the chief characteristics of each. 

Check Your Understanding 

• What are the basic characteristics of life? 

• What are the four main classes of organic molecules that are building blocks of life? 

Introduction 

When you see an organism that you've never seen before, you probably automatically classify it into a 
specific group. If it's green and leafy, you would probably call it a plant. If it's long and slithers, you would 
probably classify it as a snake. How do you make such assignments? You look at the physical features of 
the organism and think about what it has in common with other organisms. Scientists do the same thing 
when they classify living things. But scientists classify organisms not only by their physical features, but also 
by their evolutionary history and relatedness. Lions and tigers look like each other more than they look like 
bears. But it's not just appearance. The two cats are actually more closely related to each other than to 
bears. How related organisms are is an important basis for classifying them. 

Classifying Organisms 

People have been concerned with classifying organisms back to the time of the Greeks and Romans. The 
Greek philosopher Aristotle developed a classification system that divided living things into several groups 
that we still use today, including mammals, insects, and reptiles. Carl Linnaeus (1 707-1 778) (Figure 1 ) built 
on Aristotle's work to produce his own extensive classification system and invented the way we name organ- 



45 



isms by their genus and species. For example, a coyote's species name is Canis latrans. "Latrans" is the 
species and "canis" is the genus, a larger group that includes dogs, wolves, and other doglike animals. 
Linnaeus is considered the inventor of modern taxonomy, the science of naming and grouping organisms. 
He was especially interested in plants, and he used differences in flowers to classify each plant into groups. 
Modern taxonomists have reordered many groups of organisms since Linnaeus. The main categories biologists 
use are listed here from the most specific to the broadest category (Figure 2). In other words, there are 
many species in each genus, many genera (plural for "genus") in each family, and so on. The broadest and 
most inclusive category is the domain. It is currently believed that there are three domains and six kingdoms. 
We will discuss these groups more later. 




Figure 1: In the 18th century, Carl Linnaeus invented the two-name system of naming organisms (genus 
and species) and introduced the most complete classification system then known. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Carl_Linnaeus.jpg, License: Public Domain) 



46 



Species 



Genus 



Family 



Order 



Class 



Phylui 



lgdor 



Domain, 



Life 



Figure 2: This diagram illustrates the classification categories for organisms, with the broadest catagory 
(Life) at the bottom, and the most specific catagory (Species) at the top. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Biological_classification_S_Pengo.svg, License: CCSA) 

But how do taxonomists decide what domain or family an organism belongs to? Like Linnaeus, they still 
look at the physical features of the organisms and group organisms that look similar together (Figure 3). 
But taxonomists also try to piece together evolutionary relationships when assigning organisms to a specific 
group. By looking at fossils, ancient remains of living things, they can tell if organisms share a recent 
common ancestor-sort of like a "grandparent" species. A common ancestor is an ancestor shared by two 
groups of organisms. For example lions and tigers share a common ancestor; both species are descended 
from an ancient cat. If two species share a recent common ancestor, it means they are closely related and 
they will be placed in the same group. 

Another way to determine evolutionary relationships is by looking for similarities or differences in organisms' 
DNA. The number of differences in two organisms' DNA can show how closely related the two organisms 
are. You might expect, for example, that human DNA is more similar to chimpanzee DNA than to bacterial 
DNA. (And it is.) How biologists determine evolutionary history will be discussed in more detail in the Evolution 
chapter. 



47 




1. Geospisa magnirostris 
3. Geospisa panrula 



2. Qeospiza Fortis 
4. Certhidea olivacea 



Finches from Galapagos Archipelago 



Figure 3: Darwin suggested that these Galapagos Island finches share a common ancestor and evolved 
different beaks because they were eating different foods. Modern research confirms this hypothesis. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Darwin%27s_finches.jpeg, License: Public Domain) 
Naming Organisms 

Carl Linnaeus recognized a need for a system of names for each species. If we just used common names, 
we would have many different names in many different languages for the same species. To solve this 
problem, Linnaeus developed binomial nomenclature, a way to give a scientific name to every organism. 
Each species receives a two-part name in which the first word is the genus (a group of species) and the 
second word refers to one species in that genus. For example, the red maple, Acer rubra, and the sugar 
maple, Acer saccharum, are both in the same genus (Figure 4). Notice that the genus is capitalized and 
the species is not, and that the whole scientific name is in italics. The names are nearly always in Latin, the 
universal language of scholars throughout European history. Sometimes, biologists use Greek or other 
words. For example, Microtus pennsylvanicus is a species of mouse in Pennsylvania and nearby states. 




48 





Figure 4: These leaves in the top and middle photographs are from two different species trees in the Acer, 
or maple, genus. One of the characteristics of the maple genus is winged seeds (bottom). 



(Sources: http://commons.wikimedia.Org/wiki/lmage:Silver_maple_5058.jpg, 
http://www.flickr.com/photos/binkley27/2571412895/, License: CC- Attribution) 



License: 



GNU-FD; 



Even though naming species is straightforward, deciding if two organisms are the same species can 
sometimes be difficult. Linnaeus defined each species by the distinctive physical characteristics shared by 
these organisms. But two members of the same species may look quite different. For example, people from 
different parts of the world sometimes look very different, but we are all the same species (Figure 5). 

So how is a species defined? A species is group of individuals that can interbreed with one another and 
produce fertile offspring; a species does not interbreed with other groups. By this definition, two species of 
animals or plants that do not interbreed are not the same species. For example, tigers and lions can mate 
in zoos and produce kittens that are half tiger and half lion. But we still consider tigers and lions separate 
species. The two cats look and behave differently and are not known to interbreed in the wild, even though 
they can. Groups of lions and tigers do not interbreed. 



49 




Figure 5: These children are all members of the same species, Homo sapiens. 

(Source: http://www.flickr.com/photos/angela7/294746254/, License: CC-Attribution) 

Domains of Life 

All life can be divided into 3 domains: Bacteria, Archaea, and Eukarya. This is the largest and least specific 
classification, so the organisms might not look much alike, but they do have some very important traits in 
common. For example, you might be surprised that mushrooms, plants, and people are all in the same domain. 
But when you look at the cells of mushrooms, plants, and people, you will see that they do have some sim- 
ilar features. They are all eukaryotic organisms, or in the domain Eukarya. The other two domains are 
composed of prokaryotic organisms. Prokaryotic and eukaryotic cells will be discussed in the chapter titled 
Cells and Their Functions . 




50 





Figure 6: Examples from the three domains of life: Bacteria (top), Archaea (center) , and Eukarya (bottom). 

(Sources: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Hal0bacteria.jpg, License: Public Domain; http://com- 
mons.wikimedia.org/wiki/lmage:Entercoccus_sp2_lores.jpg, License: Public Domain; 

http://www.flickr.eom/photos/7326810@N08/1478892479/, License: CC_Attribution) 

Table 1: Three domains of life: Bacteria, Archaea, and Eukarya 





Archaea 


Bacteria 


Eukarya 


Multicelluar 


No 


No 


Yes 


Cell Wall 


Yes, without 
peptidoglycan 


Yes, with pepti- 
doglycan 


Varies. Plants and 
fungi have a cell 
wall; animals do 
not. 


Nucleus (DNA 
inside a mem- 
brane) 


No 


No 


Yes 



51 



Organelles in- 
side a mem- 
brane 



No 



No 



Yes 



All the cells in the domain Eukarya keep their DNA inside a membrane, a structure called the nucleus. The 
cells of other domains have DNA, but it is not inside a nucleus. The domain Eukarya is made up of four diverse 
kingdoms: plants, fungi, animals, and protists. 

Plants, such as trees and grasses, survive by capturing energyfromthesun, a process called photosynthesis. 
Animals survive by eating other organisms or the remains of other organisms. Animals range from tiny worms 
to insects, dogs, and the largest dinosaurs and whales. Fungi, such as mushrooms and molds, also survive 
by eating other organisms or the remains of other organisms. The last group listed here are the protists. 
Protists are not all descended from a single common ancestor in the way that plants, animals, and fungi 
are. Protists are a sort of miscellaneous group; they are all the organisms that are not something else. 
Protists are a diverse group of organisms that include many kinds of microscopic one-celled organisms, 
such as algae and plankton, but also giant seaweeds that can grow to be 200 feet long. Plants, animals, 
fungi, and protists might seem very different, but remember that if you look through a microscope, you would 
find cells with a membrane-bound nucleus in all them. 




«^K 



Figure 7: This microscopic alga is a protist in the domain Eukarya. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Pediastrumb0ryanum.jpg, License: Public Domain) 

The cells of the two other domains - the Archaea and the Bacteria - do not have a nucleus. All the cells in 
both domains are tiny, microscopic one-celled organisms that can reproduce without sex by dividing in two. 
The difference between the archaea and the bacteria is in their cell walls. Also, archaea often live in extreme 
environments like hot springs, geysers, and salt flats, while bacteria are abundant and live almost everywhere. 
A teaspoon of soil can contain 100 million to a billion individual bacteria. Bacteria obtain energy in lots of 
different ways. Some infect plants and animals and cause disease. Others break down dead organisms. 
The cyanobacteria photosynthesize, like plants. In fact, the ancestors of today's cyanobacteria invented 
photosynthesis more than two billion years ago. 

Viruses 

We have all heard of viruses. The flu and many other diseases are caused by viruses. But what is a virus? 
Based on the material presented in this chapter, are viruses living? No. 

A virus is essentially nucleic acid surrounded by protein. It is not made of a cell; it does not metabolize, it 
does not maintain homeostasis. Viruses need to infect a host cell to reproduce; they cannot reproduce on 
their own. However, viruses do evolve. So a virus is very different than any of the organisms that fall into 
the three domains of life. 



52 



-Genome 

(DNAorRNA) 





Icosohedral Head 



Tail Fibers 

I." 



Figure 5: These "moon lander" shaped complex virus infects Escherichia coli bacteria. 

(Source: http://en.wikipedia.Org/wiki/lmage:Tevenphage.png, Image by: User: Adenosine, License: CC-BY- 
SA2.5) 

Lesson Summary 

• Scientists have defined several major categories for classifying organisms: domain, kingdom, phylum, 
class, order, family, genus, and species. 

• The scientific name of an organism consists of its genus and species. 

• Scientists classify organisms according to their evolutionary histories and how related they are to one 
another - by looking at their physical features, the fossil record, and DNA sequences. 

• All life can be classified into three domains: Bacteria, Archaea, and Eukarya. 
Review Questions 

1. Who designed modern classification and invented the two-part species name? (Beginning) 

2. In what domain are humans? (Challenging) 

3. Quercus rubra is the scientific name for the red oak tree. What is the red oak's genus? (Intermediate) 

4. In what domain are mushrooms? (Challenging) 

5. Is it possible for organisms in two different classes to be in the same genus? (Challenging) 

6. How are organisms given a scientific name? (Beginning) 

7. Define a species. (Beginning) 

8. What kingdoms make up the domain Eukarya? (Intermediate) 

9. What is the name for the scientific study of naming and classifying organisms? (Beginning) 

10. What information do scientists use to classify organisms? 



53 



11. If molecular data suggests that two organisms have very similar DNA, what does that say about their 
evolutionary relatedness? (Challenging) 

12. Can two different species ever share the same scientific name? (Intermediate) 

13. If two organisms are in the same genus, would you expect them to look much alike? (Challenging) 

Further Reading I Supplemental Links 

• http://www.ucmp.berkeley.edu/history/linnaeus.html 

• http://www.physicalgeography.net/fundamentals/9b.html 

• http://www.pbs.org/wgbh/nova/orchid/classifying.html 

• http://en.wikipedia.org/ 

Review Answers 

1 . Carl Linnaeus 2. Eukarya 3. Quercus 4. Eukarya 5. No. If they are in the same genus then they must also 
be in the same class. 6. Binomial nomenclature. The first word is the genus and the second word is specific 
to the species. 7. Groups of organisms that can mate with one another to produce fertile offspring and do 
not mate with other such groups. 8. Fungi, plants, animals, and protists 9. Taxonomy 10. Physical features 
and their evolutionary history and relatedness. 11. The two organisms had a recent common ancestor. 12. 
Each species has a unique name. 13. Yes, as genus is one of the more specific classification catagories 
(species is the most specific), organisms in the same genus would have many features in common. 

Vocabulary 

archaea Microscopic one-celled organisms with no nucleus that tend to live in extreme 

environments. 

bacteria Microscopic one-celled organisms with no nucleus that live everywhere. 

binomial nomenclature The system for naming species in which the first word is the genus and the second 

word is the species. 

cyanobacteria Photosynthetic bacteria. 

DNA Deoxyribonucleic acid; Nucleic acid molecule that stores the genetic information. 

Eukarya Domain in which cells have a nucleus that includes plants, animals, fungi, and 

protists. 

fossils Ancient remains of living things; includes bone, teeth, and impressions. 

nucleus Tiny structure inside of some cells that walls off the DNA from the rest of the cell; 

DNA wrapped inside a membrane. 

species Group of organisms that can mate with one another to produce fertile offspring 

but do not mate with other such groups. 

taxonomy The science of naming and classifying organisms. 

Points to Consider 

• This lesson introduced the diversity of life on Earth. Do you think it is possible for cells from different or- 
ganisms to be similar even though the organisms look different? 

• Do you think human cells are different from bacterial cells? 

• Do you think it is possible for a single cell to be a living organism? 



54 



3. Cells and Their Structures 



Introduction to Cells 

Lesson Objectives 

• Explain how cells are observed. 

• Recall the cell theory. 

• Explain the levels of organization in an organism. 

Check Your Understanding 

• What are the main characteristics of living things? 

• Name the four main classes of organic molecules that are building blocks of life. 

Introduction 

How do lipids, carbohydrates, proteins, and nucleic acids come together to form a living organism? By 
forming a cell. These organic compounds are the raw materials needed for life, and a cell is the smallest 
unit of an organism that is still considered living. Cells are the basic units that make up every type of organism. 
Some organisms, like bacteria, consist of only one cell. Other organisms, like humans, consist of trillions of 
specialized cells working together. Even if organisms look very different from each other, if you look close 
enough you'll see that their cells have much in common. 




Figure 1 : The outline of onion cells are visible under a light microscope. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Microphoto-cells-onion2.jpg, License: GNU_FDL) 

Observing Cells 

Most cells are so tiny that you can't see them without the help of a microscope. The microscopes that students 
typically use at school are light microscopes. Robert Hooke created a primitive light microscope in 1 665 and 
observed cells for the very first time. Although the light microscope opened our eyes to the existence of 
cells, they are not useful for looking at the tiniest components of cells. Many structures in the cell are too 



55 



small to see with a light microscope. 

When scientists developed more powerful microscopes in the 1950s, the field of cell biology grew rapidly. 
A light microscope passes a light beam through a specimen, but the more powerful electron microscope 
passes a beam of electrons through the specimen, allowing a much closer look at the cell. 

Transmission electron microscopes (TEM), which pass an electron beam through something, are used 
to look at a very thin section of an organism and allow us to study the internal structure of cells. Scanning 
electron microscopes (SEM), which pass a beam of electrons across the surface of something, show the 
details of the shapes of surfaces, giving a 3D image. 

Electron microscopes showed many small structures in the cell that had been previously invisible with light 
microscopes. One drawback to using an electron microscope is that it only images dead cells. A light micro- 
scope can be used to study living cells. 



Figure 2: An electron microscope allows scientists to see much more detail than a light microscope, as with 
this sample of pollen. But a light microscope allows scientists to study living cells, 

(Source: http://commons.wikimedia.Org/wiki/lmage:SEM_blood_cells.jpg, License: Public Domain) 

Cell Theory 

In 1858, after microscopes had become much more sophisticated than Hooke's first microscope, Rudolf 
Virchow proposed that cells only came from other cells. For example, bacteria are composed of only one 
cell (Figure 3) and divide in half to replicate themselves. In the same way, your body makes new cells by 
the division of cells you already have. In all cases, cells only come from pre-existing cells. 

This concept is central to the cell theory. The cell theory states that: 

1 . All organisms are composed of cells. 

2. Cells are alive and the basic living units of organization in all organisms. 

3. All cells come from other cells. 

As with other scientific theories, the cell theory has been supported by thousands of experiments. And, since 
Virchow introduced the cell theory, no evidence has ever contradicted it. 



56 




Figure 3: Bacteria (pink) are an example of an organism consisting of only one cell. 

(Source: http://www.flickr.eom/photos/83371410@N00/1644090403/, License: CC Attribution) 

Levels of Organization 

Although cells share many of the same features and structures, as we will discuss in the next section, they 
also can be quite different. Each cell in your body is specialized for a specific task. For example: 

• Red blood cells are shaped with a pocket to increase their surface area for absorbing and releasing 
oxygen. 

• Nerve cells, which can quickly transmit the sensation of touching a hot stove to your brain, are elongated 
and stringy to allow them to form a complex network with other nerve cells. 

• Skin cells are flat and fit tightly together. 

As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms 
have many types of specialized cells in their bodies. 



57 




Figure 4: Red Blood cells are specialized to carry oxygen in the blood. 

(Source: http://commons.wikimedia.org wiki/lmage:SEM_blood_cells.jpg, License: Public Domain) 




Figure 5: Neurons are shaped to conduct electrical impulses to many other nerve cells. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Pyramidal_hippocampal_neuron_40x.jpg, License: CC- 
SA) 



58 




Figure 6:These epidermal cells make up the "skin" of plants. Note how the cells fit tightly together. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Rhoeo_Discolor_epidermis.jpg, License: GNU- FD) 

While cells are the basic units of an organism, groups of specialized cells can be organized into tissues. For 
example, your liver cells are organized into liver tissue, which is organized into an organ, your liver. Organs 
are formed from two or more specialized tissues working together for a common function. All organs, from 
your heart to your liver, are made up of an organized group of tissues. 

These organs are part of a larger organization pattern, the organ systems. For example, your brain works 
together with your spinal cord and other nerves to form the nervous system. This organ system must be 
organized with other organ systems, such as the circulatory system and the digestive system, for your body 
to work. Organ systems are coordinated together to form the complete organism. As you can see, there are 
many levels of organization in living things. 




Figure 7: Levels of Organization, from the atom to the organism. 

Lesson Summary 

• Cells were first observed under the light microscope, but today electron microscopes allow scientists to 
take a closer look at the internal structures of cells 



59 



• The Cell Theory says that 1 ) all organisms are composed of cells; 2) cells are alive and the basic living 
units of organization in all organisms; and 3) All cells come from other cells. 

• Cells are organized into tissues, which are organized into organs, which are organized into organ systems, 
which are organized to create the whole organism. 

Review Questions 

1 . What type of microscope would you use to study living algae cells? (Intermediate) 

2. What type of microscope would you use to study the details on the surface of a cell? (Intermediate) 

3. What type of microscope would be best for studying internal structures of cells? (Intermediate) 

4. According to the cell theory, can we synthesize a cell in the laboratory from organic molecules? (Chal- 
lenging) 

5. Do all cells work exactly the same? (Intermediate) 

6. Put the following in the correct order from simplest to most complex: organ, cell, tissue, organ system. (Be- 
ginning) 

Further Reading I Supplemental Links 

Baeuerle, Patrick A. and Landa, Norbert. The Cell Works: Microexplorers. Barron's; 1997, Hauppauge, New 
York. 

Sneddon, Robert. The World of the Cell: Life on a Small Scale. Heinemann Library; 2003, Chicago. 

Wallace, Holly. Cells and Systems. Heinemann Library; 2001, Chicago. 

Vocabulary 

cell The smallest unit of an organism that is still considered living; the basic 

unit that make up every type of organism. 

organ A group of tissues that work together to perform a common function. 

organ system A group of organs that work together to perform a common function. 

scanning electron microscope Microscope that scans the surface of a tissue or cell, showing a 3D 
(SEM) image. 

tissue A group of specialized cells that function together. 

transmission electron micro- Microscope used to look at a very thin section of an organism and allow 
scope (TEM) us to study the internal structure of cells. 

Review Answers 

1. Light microscope 

2. Scanning electron microscope 

3. Transmission electron microscope 

4. No, cells can only come from other cells. 

5. No, cells are specialized for specific tasks. 



60 



6. Cell, tissue, organ, organ system 

Points to Consider 

• Do you think there would be a significant difference between bacteria cells and your brain cells? What 
might they be? 

• Do you think a bacteria cell and brain cell have some things in common? What might they be? 

• Do you think cells are organized? What would be the benefit of organization? 

Cell Structures 

Lesson Objectives 

• Compare prokaryotic and eukaryotic cells. 

• List the organelles of the cell and their functions. 

• Discuss the structure and function of the cell membrane and cytosol. 

• Describe the structure and function of the nucleus. 

• Distinguish between plant and animal cells. 

Check Your Understanding 

• What is a cell? 

• How do we visualize cells? 

Introduction 

Understanding the structure and function of cells is essential to understanding how living organisms work. 
Cell biology is central to all other fields of biology, including medicine. Many human diseases and disorders 
are caused by the malfunction of people's cells. Furthermore, toxins in the environment often act on specific 
cellular processes. The healthy functioning of the body and its organs is dependent on its smallest unit -the 
cell. 

To better understand the biology of the cell, you will first learn to distinguish the two basic categories of all 
cells: prokaryotic and eukaryotic cells. You will also learn what makes a cell specialized; there are major 
differences between a "simple" cell, like a bacteria, and a "complex" cell, like a cell in your brain. To understand 
these differences, you need to first understand the basic components of the cell, which include the: 

• Cell membrane 

• Nucleus and chromosomes 

• Other organelles 

Prokaryotic and Eukaryotic Cells 

There are two basic types of cells, prokaryotic cells, which include bacteria and archaea, and eukaryotic 

cells, which include all other cells. Prokaryotic cells are much smaller and simpler than eukaryotic cells; 
eukaryotic cells can be considered to be "specialized." Prokaryotic cells are surrounded by a cell wall that 
supports and protects the cell. In prokaryotic cells the DNA, the genetic material, forms a single large circle 
that coils up on itself. Prokaryotic cells also can contain extra small circles of DNA, known as plasmids. 
The two types of organisms consisting of prokaryotic cells belong to the domain Bacteria and the domain 



61 



Archaea. These two domains were discussed in the Introduction to Living Things chapter. 

lmage:Lsc-0302-01.prokaryotepng|550px 

Figure 1 : Prokaryotes do not have a nucleus. Instead, their genetic material is a simple loop of DNA. 

(Source: http://commons.wikimedia.org /wiki/lmage:Prokaryote_cell_diagram_no.svg, License: Public Domain) 

The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells store their DNA in a 
membrane-enclosed nucleus. The presence of a nucleus is the primary distinguishing feature of a eukaryotic 
cell. In addition to the nucleus, eukaryotic cells have other subcompartments, small membrane-enclosed 
structures called organelles. Membrane-enclosed organelles and a nucleus are absent in prokaryotic cells. 
Eukaryotic cells include the cells of fungi, animals, protists, and plants. 



Nucleus 

Nuclear pore 
Nuclear envelope 
Chromatin 
Nucleolus 
Ribosomes 



Golgi vesicles 
(golgj apparatus) 



Lysosome 

Centrioles 



Cytoplasm 




Plasma membrane 

Mitochondrion 

Peroxisome 

Cytoskeleton 

Free Ribosomes 



Secretory vesicle 



Figure 2: Eukaryotic cells contain a nucleus (where the DNA "lives," and surrounded by a membrane) and 
various other special compartments surrounded by membranes, called "organelles." For example, notice in 
this image the mitochondria, lysosomes, and peroxisomes. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Animal_cell_structure.svg, License: Public Domain) 

Table 1: Comparison of Prokaryotic and Eukaryotic Cells 



Feature 


Prokaryotic cells 


Eukaryotic cells 


DNA 


Single "naked" circle; plasmids 


In membrane-enclosed nucleus 


Membrane-enclosed organelles 


No 


Yes 


Examples 


Bacteria 


Plants, animals, fungi 



The Plasma Membrane and Cytosol 

Both eukaryotic and prokaryotic cells have a plasma membrane. The plasma membrane is a double layer 
of specialized lipids, known as phospholipids, along with many special proteins. The function of the plasma 
membrane, also known as the "cell membrane," is to control what goes in and out of the cell. Some molecules 
can go through the cell membrane in and out of the cell and some can't, so biologists say the membrane is 
semi-permeable. It is almost as if the membrane chooses what enters and leaves the cell. 



62 



The cell membrane gives the cell an inside that is separate from the outside world. Without a cell membrane, 
the parts of a cell would just float away. A cell needs a boundary even more than we need our skin. Without 
a cell membrane, a cell would be unable to maintain a stable internal environment separate from the external 
environment, what we call homeostasis. You can learn more about cell membranes in the Cell Functions 
chapter. 

Eukaryotic and prokaryotic cells also share an internal fluid-like substance called the cytosol. The cytosol 
is composed of water and other molecules, including enzymes that speed up the cell's chemical reactions. 
Everything in the cell - the nucleus and the organelles - sit in the cytosol. The term cytoplasm refers to the 
cytosol and all the organelles, but not the nucleus. 

Table 2: Some Eukaryotic Organelles 



Organelle 


Function 


Ribosomes 


Involved in making 
proteins 


Golgi apparatus 


Packages proteins 
and some polysac- 
charides 


Mitochondria 


Makes ATP 


Smooth ER 


Makes lipids 


Chloroplast 


Makes sugar (photo- 
synthesis) 


Lysosomes 


Digests macro- 
molecules 



The Nucleus and Chromosomes 

The nucleus, which is found exclusively in eukaryotic cells, is a membrane-enclosed structure that contains 
most of the genetic material of the cell. Like a library, it holds vital information, mainly detailed instructions 
for building proteins. The nuclear envelope, a double membrane that surrounds the nucleus, controls which 
molecules go in and out of the nucleus. 

Inside the nucleus are the chromosomes, the DNA all wrapped in special proteins. The genetic information 
on the chromosomes is stored made it available to the cell when necessary and also duplicated when it is 
time to pass the genetic information on when a cell divides. All the cells of a species carry the same number 
of chromosomes. For example, human cells each have 23 pairs of chromosomes. Each chromosome in 
turn carries hundreds or thousands of genes that encode proteins that help determine traits as varied as 
tooth shape, hair color, or kidney function. 



63 



nvelope 
Outer membrane 
Inner membrane ^*-^ 


^^^^^P^^^^ta 


Nucleolus jf 




Nucleoplasm .rfi^^^- 




Het^roch^rtiatift - — V- 




Euchro matin — — ■ Xs. 

v r / 




Piboson^ei<^' c / r 


r 


r ° 

Nuclear pore 






Figure 3: In eukaryotic cells, the DNA is kept in a nucleus. The nucleus is surrounded by a double plasma 
membrane called the nuclear envelope. Within the nucleus is the nucleolus (smaller yellow ball). 



(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Diagram_human_cell 
Domain) 



nucleus. svg, License: Public 



The Cell Factory 

Just as a factory is made up of many people, machines, and specific areas, each part of the whole playing 
a different role, a cell is also made up of different parts, each with a special role. For example, the nucleus 
of a cell is like a safe containing the factory's trade secrets, including how to build thousands of proteins, 
how much of each one to make, and when. The mitochondria are powerhouses that generate the ATP 
needed to power chemical reactions. Plant cells have special organelles called chloroplasts that capture 
energy from the sun and store it in the chemical bonds of sugar molecules - in the process called photosyn- 
thesis. (The cells of animals and fungi do not photosynthesize and do not have chloroplasts.) 

The vacuoles are storage centers, and the lysosomes are the recycling trucks that carry waste away from 
the factory. Inside lysosomes are enzymes that break down old molecules into parts that can be recycled 
into new ones. Eukaryotic cells also contain and internal skeleton called the cytoskeleton. Like our bony 
skeleton, a cell's cytoskeleton gives the cell a shape and helps it move parts of the cell. 

In both eukaryotes and prokaryotes, ribosomes are where proteins are made. Some ribosomes cluster on 
folded membranes called the endoplasmic reticulum (ER). If the ER is covered with ribosomes, it looks 
bumpy and is called rough ER. If the ER lacks ribosomes, it is smooth and is called smooth ER. Proteins 
are made on rough ER and lipids are made on smooth ER. 

Another set of folded membranes in cells is the Golgi apparatus, which works like a mail room. The Golgi 
apparatus receives the proteins from the rough ER, puts sugar molecule "shipping addresses" on the proteins, 
packages them up in vesicles, and then sends them to the right place in the cell. 



64 




(stack of thylakoids) 



Thylakoid 



Lumen 
(inside of thylakoid) 




Figure 4: Diagram of chloroplast (top) and electron microscope image of two mitochondria (bottom). 
Chloroplasts and mitochondria provide energy to cells. If the bar at the bottom of the electron micrograph 
image is 200 nanometers, what is the diameter of one of the mitochondria? 

(Sources: http://commons.wikimedia.Org/wiki/lmage:Mitochondria%2C_mammalian_lung_-_TEM 

_%282%29.jpg, http://commons.wikimedia.0rg/wiki/lmage:Chloroplast-new.jpg, License: Public Domain) 

Plant Cells 

Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells. First, 
plant cells are unique in having a large central vacuole that holds a mixture of water, nutrients, and wastes. 
A plant cell's vacuole can make up 90% of the cell's volume. In animal cells, vacuoles are much smaller. 

Second, plant cells have a cell wall, which animal cells do not. A cell wall gives the plant cell strength, 
rigidity, and protection. Although bacteria and fungi also have cell walls, a plant cell wall is made of a different 
material. Plant cell walls are made of the polysaccharide cellulose, fungal cell walls are made of chitin, and 
bacterial cell walls are made of peptidoglycan. 



65 




Figure 5: A plant cell has several features that make it different from an animal cell, including a cell wall, 
huge vacuoles, and several kinds of plastids, including chloroplasts (which photosynthesize). 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Plant_cell_structure_svg.svg, License: Public Domain) 




Figure 6: In this photo of plant cells taken with a light microscope, you can see a cell wall (purple) around 
each cell and green chloroplasts. 

(Source: ttp://en.wikipedia.org/wiki/lmage:Chloroplasten.jpg, License: CC- Attribution) 

A third difference between plant and animal cells is that plants have several kinds of organelles called 
plastids. There are several kinds of plastids, including chloroplasts, needed for photosynthesis; leucoplasts, 
which store starch and oil; and brightly colored chromoplasts, which give some flowers and fruits their 
yellow, orange, or red color. You will learn more about chloroplasts and photosynthesis in the chapter titled 
Cell Functions. 

Lesson Summary 

• Prokaryotic cells lack a nucleus; eukaryotic cells have a nucleus. 

• Each component of a cell has a specific function. 



66 



• Plant cells have unique features including plastids, cell walls, and central vacuoles. 

Review Questions 

1 . What are the two basic types of cells? (Beginning) 

2. What are organelles? (Beginning) 

3. Discuss the main differences between prokaryotic cells and eukaryotic cells. (Challenging) 

4. What is the plasma membrane and what is its role? (Challenging) 

5. What organelle is known as the "powerhouse" of the cell? (Beginning) 

6. Why does photosynthesis not occur in animal cells? (Intermediate) 

7. What are the main differences between a plant cell and an animal cell? (Intermediate) 

Further Reading I Supplemental Links 

Baeuerle, Patrick A. and Landa, Norbert. The Cell Works: Microexplorers. Barron's; 1997, Hauppauge, New 
York. 

Sneddon, Robert. The World of the Cell: Life on a Small Scale. Heinemann Library; 2003, Chicago. 

Wallace, Holly. Cells and Systems. Heinemann Library; 2001, Chicago. 

Vocabulary 



cell 

cell wall 
central vacuole 
chloroplast 

chromosome 

cytoplasm 

cytoskeleton 

cytosol 

endoplasmic reticulum 
(ER) 

eukaryotic cell 

golgi apparatus 

homeostasis 

lysosome 
mitochondria 



The smallest unit of an organism that is still considered living; the basic unit 
that make up every type of organism. 

Provides strength and protection for the cell; found around plant, fungal, and 
bacterial cells. 

Large organelle containing water, nutrients, and wastes that can take up to 
90% of a plant cell's volume. 

Green organelle that captures solar energy and stores the energy in sugars 
through the process of photosynthesis; chloroplasts are found only in cells 
that perform photosynthesis. 

The cell structure in eukaryotic cells containing the genes; made of DNA and 
protein. Human cells have 23 pairs of chromosomes. 

All the contents of the cell besides the nucleus, including the cytosol and the 
organelles. 

The internal scaffolding of the cell; maintains the cell shape and aids in moving 
the parts of the cell. 

A fluid-like substance inside the cell; organelles are embedded in the cytosol. 

A folded membrane organelle; rough ER modifies proteins and smooth ER 
makes lipids. 

Cell belonging to the domain Eukarya (fungi, animals, protists, and plants); 
has a membrane-enclosed nucleus and various organelles. 

The organelle where proteins are modified, labeled, packaged into vesicles, 
and shipped. 

The ability to maintain a stable internal environment separate from the external 
environment. 

Organelle which contains enzymes that break down unneeded materials. 

The organelle in all eukaryotic cells that makes adenosine triphosphate (ATP), 
the "energy currency" of cells. 



67 



nuclear envelope A double membrane that surrounds the nucleus; helps regulate the passage 

of molecules in and out of the nucleus. 

nucleus Membrane enclosed organelle in eukaryotic cells that contains the DNA; pri- 

mary distinguishing feature between a eukaryotic and prokaryotic cell; the in- 
formation center, containing instructions for making all the proteins in a cell, 
as well as how much of each one. 

organelle Small structure wrapped in a membrane found only in eukaryotic cells; mito- 

chondria, plastids, and vacuoles, for example. A ribosome is not technically 
an organelle, because it is not enclosed in a membrane. 

plasma membrane Surrounds the cell; made of a double layer of specialized lipids, known as 

phospholipids, with embedded proteins; regulates the movement of substances 
into and out of the cell; also called the cell membrane. 

plasmid Small circular piece of DNA; found in prokaryotic cells. 

prokaryotic cell Cell with no nucleus or other membrane-enclosed organelles; bacteria and 

archaea. 

ribosome The cell structure on which proteins are made; not surrounded by a membrane; 

found in both prokaryotic and eukaryotic cells. 

rough endoplasmic reticu- The part of the ER with ribosomes attached; proteins can be modified in the 
lum rough ER before they are packed into vesicles for transport to the golgi appa- 

ratus. 

semi-permeable allowing only certain materials to pass through; characteristic of the cell 

membrane. 

smooth endoplasmic Part of the ER that does not have ribosomes attached; where lipids are syn- 

reticulum thesized. 

vesicle Small membrane-enclosed sac; transports proteins around a cell or out of a 

cell. 

Review Answers 

1 . prokaryotic and eukaryotic 

2. Organelles are compartments within a cell where specilized functions occur. 

3. Prokaryotic cells are much smaller and simpler than eukaryotic cells; eukaryotic cells can be considered 
to be "specialized" and contain membrane-bound organelles. Prokaryotic cells are surrounded by a cell 
wall. In prokaryotic cells the DNA forms a single large circle that coils up on itself. Eukaryotic cells have 
a nucleus with numerous chromosomes. 

4. The plasma membrane is a double layer of phospholipids lipids along with many special proteins. The 
function of the plasma membrane is to control what goes in and out of the cell. The plasma membrane 
gives the cell an inside that is separate from the outside world. 

5. mitochondria 

6. As animal cells lack chloroplasts, the organelle in which photosynthesis occurs, this process cannot occur 
in animal cells. 

7. First, plant cells have a large central vacuole that holds a mixture of water, nutrients, and wastes. Second, 
plant cells have a cell wall, which animal cells do not. Third, plant cells have chloroplasts. 

Points to Consider 

• Think about what molecules would need to be transported into cells. 

• Discuss why it would be important for some molecules to be kept out of a cell. 



68 



4. Cell Functions 



Transport 

Lesson Objectives 

• Describe several methods of transporting molecules and ions into and out of the cell. 

• Distinguish between active and passive transport. 

• Explain how diffusion and osmosis work. 

Check Your Understanding 

• What structure surrounds the cell? 

• What is the primary component of the cell membrane? 

• What does homeostasis mean? 

Introduction 

All organisms and their cells need to maintain homeostasis. But how can a cell keep a stable internal envi- 
ronment when the environment around the cell is constantly changing? Obviously, the cell needs to separate 
itself from the external environment. This job is accomplished by the cell membrane. The cell membrane is 
selectively permeable, or "semipermeable," which means that only some molecules can get through the 
membrane. If the cell membrane was completely permeable, the inside of the cell would be about the same 
as the outside and the cell could not achieve homeostasis. 

How does the cell maintain this selective permeability? How does the cell control what molecules enter and 
leave the cell? The ways that cells control what passes through the cell membrane will be the focus of this 
lesson. 

What is Transport? 

The selectively permeable nature of the plasma membrane is due in part to the chemical composition of the 
membrane. Recall that the membrane is a double layer of phospholipids (a "bilayer") embedded with proteins 
(Figure 1). A single phospholipid molecule has a hydrophilic, or water-loving, head and hydrophobic, or 
water-fearing, tail. The hydrophilic heads face the inside and outside of the cell, where water is abundant. 
The water-fearing, hydrophobic tails face each other in the middle of the membrane. At body temperature, 
the plasma membrane is fluid and constantly moving, like a soap bubble; it is not a solid structure. 

Water and small non-charged molecules such as oxygen and carbon dioxide can pass freely through the 
membrane by slipping around the phospholipids. But larger molecules and charged molecules cannot pass 
through the plasma membrane easily. Therefore, special methods are needed for transporting some molecules 
across the plasma membrane and into or out of the cell. 



69 



Protein channel 
f (transport protein) 




Phospholipid bilayer 



Figure 1: The plasma membrane is made up of a phospholipid bylayerwith embedded proteins. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Cell_membrane_detailed_diagram.svg, License: Public 
Domain) 

Since atoms have an equal number of protons and electrons, they have no net 
charge . The negative charges of the electrons balance out the positive charges 
of the protons . Many molecules have an equal number of electrons and protons , 
so we call them non-polar molecules. However, some atoms can lose or gain 
electrons easily, giving them a positive or negative charge. These charged 
particles are called ions. If an atom loses an electron, it becomes a posi- 
tively charged ion, such as the sodium ion Na + . If an atom gains an electron, 
it will be a negatively charged ion, such as the chloride ion, CI ~ . Na + and 

CI " readily form NaCl, or common table salt. Since Na + and CI " are charged, 
they are unable to pass freely through the plasma membrane. 



Passive Transport 

Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is 
the movement of molecules from an area where there is a higher concentration (larger amount) of the sub- 
stance to an area where there is a lower concentration (lower amount) of the substance. The amount of a 
substance in relation to the volume, is called concentration. Diffusion requires no energy input from the 
cell. Diffusion occurs by the random movement of molecules; molecules move in both directions (into and 
out of the cell), but there is a greater movement from an area of higher concentration towards an area of 
lower concentration. The movement of the substance from a greater concentration to a lesser concentration 
is referred to as moving down the concentration gradient. For example, oxygen diffuses out of the air sacs 
in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood. 
Oxygen moves down the concentration gradient from your lungs into your bloodstream 



70 




semipermeable membrane 



Figure 2: Diffusion across a membrane does not require an input of energy. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Diffusion.en.jpg, License: Public Domain) 

The diffusion of water across a membrane due to concentration differences is called osmosis. If a cell is 
placed in a hypotonic solution, meaning the solution has a lower concentration of dissolved material than 
what is inside the cell, water will move into the cell. This causes the cell to swell, and it may even burst. 
Organisms that live in fresh water, which is a hypotonic solution, have to prevent too much water from 
coming into their cells. Freshwater fish excrete a large volume of dilute urine to rid their bodies of excess 
water. 

If a cell is placed in a hypertonic solution, meaning there is more dissolved material in the outside environ- 
ment than in the cell, water will leave the cell. That can cause a cell to shrink and shrivel. Marine animals 
live in salt water, which is a hypertonic environment; there is more salt in the water than in their cells. To 
prevent losing too much water from their bodies, these animals intake large quantities of salt water and secrete 
salt by active transport, which will be discussed later in this lesson. 

To keep cells intact, they need to be placed in an isotonic solution, a solution in which the amount of dis- 
solved material is equal both inside and outside the cell. Therefore, there is no net movement of water into 
or out of the cell. Water still flows in both directions, but an equal amount enters and leaves the cell. In the 
medical setting, red blood cells can be kept intact in a solution that is isotonic to the blood cells. If the blood 
cells were put in pure water, the solution would be hypotonic to the blood cells, so the blood cells would 
swell and burst. 



Hypertonic 



Isotonic 



Hypotonic 




Figure 3: Osmosis causes these red blood cells to change shape by losing or gaining water. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Osmotic_pressure_on_blood_cells_diagram.svg, License: Public 
Domain) 

Sometimes diffusion across the membrane is slow or even impossible for some charged or large molecules. 
These molecules need the help of special helper proteins that are located in the plasma membrane. Ion 



71 



channel proteins move ions across the plasma membrane. Other molecules, such as glucose, move across 
the cell membrane by facilitated diffusion, in which a carrier protein physically moves the molecule across 
the membrane (Figure 4). Both channel proteins and carrier proteins are specific for the molecule transported. 
Movement by ion channel proteins and facilitated diffusion are still considered passive transport, meaning 
they move molecules down the cell's concentration gradient and do not require any energy input. 




Figure 4: Facilitated Diffusion is a type of passive transport where a carrier protein aids in moving the 
molecule across the membrane. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Scheme_facilitated_diffusi0n_in_cell_membrane-en.svg, 
License: Public Domain) 

Active Transport 

During active transport, molecules move against the concentration gradient, toward the area of higher con- 
centration. This is the opposite of diffusion. Active transport requires both an input of energy, in the form of 
ATP, and a carrier protein to move the molecules. These proteins are often called pumps, because, as a 
water pump uses energy to force water against gravity, proteins involved in active transport use energy to 
move molecules against their concentration gradient. 

There are many examples of why active transport is important in your cells. One example occurs in your 
nerve cells. In these cells, the sodium-potassium pump (Figure 5) moves sodium outside the cell and 
potassium into the cell, both against their concentration gradients. 



Extracellular space 



• o 

/Hi wi )rn)mfrn 

Cell membrane 

TOOT5 



1 Sodium 
Na + 




ATp r 



Intracellular space 




Figure 5: The sodium-potassium pump moves sodium ions to the outside of the cell and potassium ions to 
the inside of the cell. ATP is required for the protein to change shape. As ATP adds a phosphate group to 
the protein, it leaves behind adenosine diphosphate (ADP). 



72 



(Source: http://en.wikipedia.0rg/wiki/lmage:S0dium-P0tassium_pump.svg, License: Public Domain) 

Transport Through Vesicles 

Some large molecules are just too big to move across the membrane, even with the help of a carrier protein. 
These large molecules must be moved through vesicle formation, a process by which the large molecules 
are packaged in a small bubble of membrane for transport. This process keeps the large molecules from 
reacting with the cytoplasm of the cell. Vesicle formation does require an input of energy, however. 

There are several kinds of vesicle formation that allow large molecules to move across the plasma membrane. 
Exocytosis moves large molecules outside of the cell. During exocytosis, the vesicle carrying the large 
molecule fuses with the plasma membrane. The large molecule is then released outside of the cell, and the 
vesicle is absorbed into the plasma membrane. Endocytosis is the process by which cells take in large 
molecules by vesicle formation. Types of endocytosis include phagocytosis and pinocytosis. Phagocytosis 
moves large substances, even another cell, into the cell. Phagocytosis occurs frequently in single-celled 
organisms, such as amoebas. Pinocytosis (Figure 6) involves the movement of liquid or very small particles 
into the cell. These processes cause some membrane material to be lost as these vesicles bud off and come 
into the cell. This membrane is replaced by the membrane gained through exocytosis. 



Pinocytosis 



Extracellular fluid 




cytoplasm 
Vesicle 



Figure 6: During endocytosis, exocytosis and pinocytosis, substances are moved into or out of the cell via 
vesicle formation. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Pinocytosis.svg, License: Public Domain) 

Lesson Summary 

• The plasma membrane is selectively permeable or semi-permeable, meaning that some molecules can 
move through the membrane easily, while others require specialized transport mechanisms. 

• Passive transport methods, including diffusion, ion channels, facilitated diffusion, and osmosis, move 
molecules in the direction of the lowest concentration of the molecule and do not require energy. 

• Active transport methods move molecules in the direction of the higher concentration and require energy 
and a carrier protein. 

• Vesicles can be used to move large molecules, which requires energy input. 

Review Questions 

1 . What happens when a cell is placed in a hypotonic solution? (Challenging) 



73 



2. What happens when a cell is placed in a hypertonic solution? (Challenging) 

3. What's the main difference between active and passive transport? (Beginning) 

4. List an example of active transport. (Intermediate) 

5. List the types of passive transport. (Challenging) 

6. Why is the plasma membrane considered semipermeable? (Beginning) 

7. What is the process where a cell engulfs a macromolecule, forming a vesicle? (Intermediate) 

8. What is diffusion? (Intermediate) 

9. Explain the results of a sodium-potassium pump working across a membrane. (Challenging) 
1Q Does facilitated transport move a substance down or up a gradient? (Beginning) 

Further Reading I Supplemental Links 

http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/passive.html 

http://www.vivo.colostate.edu/hbooks/molecules/sodium_pump.html 

http://www.biologycorner.com/bio1/diffusion.html 

http://www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html 

http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/anima- 
tions/ion_pump/ionpump.html 

http://www.enwikipedia.org/ 
Vocabulary 



active transport 

concentration 
diffusion 

endocytosis 
exocytosis 

facilitated diffusion 

homeostasis 
hypertonic solution 

hypotonic solution 

ion 

ion channel 

isotonic solution 

osmosis 
passive transport 



Moving a molecule from an area of lower concentration to an area of higher 
concentration; requires a carrier protein and energy. 

The amount of a substance in relation to the volume. 

Movement of molecules from an area of high concentration to an area of low 
concentration; requires no energy. 

Movement of substances into the cell by vesicle formation. 

Movement of substances out of the cell by a vesicle fusing with the plasma 
membrane. 

Diffusion in which a carrier protein physically moves the molecule across the 
membrane; a form of passive transport. 

Maintaining a stable internal environment despite any external changes. 

Having a higher solute concentration than the cell; cell will lose water by osmo- 
sis. 

Having a lower solute concentration than the cell; cell will gain water by osmosis. 

An atom that carries a negative or positive charge. 

Protein in the plasma membrane that allows ions to pass through. 

A solution in which the amount of dissolved material is equal both inside and 
outside the cell; no net gain or loss of water. 

Diffusion of water across a membrane. 

Movement of molecules from an area of higher concentration to an area of 
lower concentration; requires no energy. 



74 



phagocytosis Movement of large substances, including other cells, into the cell by vesicle 

formation. 

phospolipid A lipid molecule with a hydrophilic head and two hydrophobic tails; makes up 

the cell membrane. 

pinocytosis Movement of macromolecules into the cell by vesicle formation. 

selectively permeable Semipermeable; property of allowing only certain molecules to pass through 

the cell membrane. 

sodium-potassium pump Carrier protein that moves sodium ions out of the cell and potassium ions into 

the cell; works against the concentration gradient and requires energy. 

vesicle formation The formation of a small membrane-bound sac that can store and move sub- 

stances into and out of the cell. 

Review Answers 

1 . Water enters the cell, and it may burst. 

2. Water leaves the cell toward he area of higher solute concentration, shriveling the cell. 

3. Active transport moves molecules or ions against the concentration gradient, while passive moves 
molecules with the gradient; active transport requires energy, while passive does not. 

4. the sodium-potassium pump, etc. 

5. channel proteins, diffusion, osmosis, facilitated transport 

6. Some substances can diffuse through the membrane freely, while others cannot. 

7. pinocytosis 

8. The movement of molecules down their concentration gradients, from a higher to a lower concentration. 

9. The inside of the cell becomes negatively charged. 

1Q Follows down the gradient, from an area of high concentration to a area of lower concentration. 

Points to Consider 

• The next lesson discusses photosynthesis. 

• It is often said that plants make their own food. What do you think this means? 

• What substances would need to move into a leaf cell? 

• What substances would need to move out of a leaf cell? 

Photosynthesis 

Lesson Objectives 

• Explain the importance of photosynthesis. 

• Write and interpret the chemical equation for photosynthesis. 



75 



• Describe what happens during the light reactions and the Calvin cycle. 

Check Your Understanding 

• How are plant cells different from animal cells? 

• In what organelle does photosynthesis take place? 

Introduction 

Almost all life on Earth depends on photosynthesis. Recall that photosynthesis is the process by which 
plants use the sun's energy to make their own "food" from carbon dioxide and water. For example, animals, 
such as caterpillars, eat plants and therefore rely on the plants to obtain energy. If a bird eats a caterpillar, 
then the bird is obtaining the energy that the caterpillar gained from the plants. So the bird is indirectly getting 
energy that began with the "food" formed through photosynthesis. Almost all organisms obtain their energy 
from photosynthetic organisms, either directly, by eating photosynthetic organisms, or indirectly by eating 
other organisms that ultimately obtained their energy from photosynthetic organisms. Therefore, the process 
of photosynthesis is central to sustaining life on Earth. 

Overview of Photosynthesis 

Photosynthesis is the process that converts the energy of the sun, or solar energy, into carbohydrates, a 
type of chemical energy. During photosynthesis, carbon dioxide and water combine with solar energy, 
yielding glucose (the carbohydrate) and oxygen. As mentioned previously, plants can photosynthesize, but 
plants are not the only organisms with this ability. Algae, which are plant-like protists, and cyanobacteria 
(certain bacteria which are also known as blue-green bacteria, or blue-green algae) can also photosynthesize. 
Algae and cyanobacteria are important in aquatic environments as sources of food for larger organisms. 

Photosynthesis mostly takes place in the leaves of a plant. The green pigment in leaves, chlorophyll, helps 
to capture solar energy. And special structures within the leaves provide water and carbon dioxide, which 
are the raw materials for photosynthesis. The veins within a leaf carry water which originates from the roots, 
and carbon dioxide enters the leaf from the air through special pores called stomata. 




Figure 1: Stomata are special pores that allow gasses to enter and exit the leaf. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Est0ma.jpg, License: GNU_FD) 

The water and carbon dioxide are transported within the leaf to the chloroplast, the organelle in which 
photosynthesis takes place. The chloroplast has two distinct membrane systems; an outer membrane sur- 
rounds the chloroplast and an inner membrane system forms flattened sacs called thylakoids. As a result, 
there are two separate spaces within the chloroplast. The interior space that surrounds the thylakoids is 
filled with a fluid called stroma. The inner compartments formed by the thylakoid membranes are called the 



76 



thylakoid space. 



Stroma 
(aqueous fluid) 




Thylakoid 



Figure 2: The chloroplast is the photosynthesis factory of the plant. 

(Source: http://cornmons.wikimedia.0rg/wiki/lmage:Chloroplast-new.jpg, License: Public Domain) 

The overall chemical reaction for photosynthesis is 6 molecules of carbon dioxide (C0 2 ) and 6 molecules 
of water (H 2 0), with the addition of solar energy, yields 1 molecule of glucose (C 6 H 12 6 ) and 6 molecules of 
oxygen (0 2 ). Using chemical symbols the equation is represented as follows: 



6C0 2 + 6H 2 



C 6 H 12 6 +60 2 



Oxygen is a byproduct of the process of photosynthesis and is released to the atmosphere through the 
stomata. Therefore, plants and other photosynthetic organisms play an important ecological role in converting 
carbon dioxide into oxygen. As you know, animals need oxygen to carry out the energy-producing reactions 
of their cells. Without photosynthetic organisms, many other organisms would not have enough oxygen in 
the atmosphere to survive. 

The overall process of photosynthesis does not happen in one step, however. The chemical equation of 
photosynthesis shows the results of many chemical reactions. The chemical reactions that make up the 
process of photosynthesis can be divided into two groups: the light reactions (also known as the light-depen- 
dent reactions, because these reactions only occur during daylight hours) and the Calvin cycle, or the light- 
independent reactions. During the light reactions, the energy of sunlight is captured, while during the Calvin 
cycle, carbon dioxide is converted into glucose, which is a type of sugar. 



77 



H 2 



I2O o : 

* llnll * 



Light reactions 





/Calvin 1 
Cycle 



I Cycle J 



sugar 



Figure 3: This overview of photosynthesis shows that light is captured during the light reactions, resulting 
in the production of ATP and the electron carrier NADPH. Through the Calvin cycle, these materials are 
used to fix carbon dioxide into sugar. Also during the Calvin cycle, NADP+ and ADP are regenerated. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Simple_photosynthesis_overview.png, License: CC-SA) 

Stage 1: Capturing Light Energy 

In the first step of the light reactions, solar energy is absorbed by the chlorophyll (and accessory pigments) 
within the chloroplast's thylakoid membranes. This absorbed energy excites electrons in the thylakoid 
membranes. The electrons are then transferred from the thylakoid membranes by a series of electron carrier 
molecules. The series of electron carrier molecules that transfers electrons is called the electron transport 
chain. During this process water molecules in the thylakoid are split to replace the electrons that left the 

pigment, releasing oxygen and adding hydrogen ions (H + ) to the thylakoid space. As the thylakoid becomes 
a reservoir for hydrogen ions, a chemiosmotic gradient forms as there are more hydrogen ions in the 

thylakoid than in the stroma. As H + ions flows from the high concentration in the thylakoid to the low concen- 
tration in the stroma, they provide energy as they pass through an enzyme called ATP synthase. ATP synthase 

uses the energy of the movement of H + ions to make ATP. Meanwhile, highly energized electrons from the 

electron transport chain combine with the electron carrier NADP + to become NADPH. NADPH will carry this 
energy in the electrons to the next phase of photosynthesis, the Calvin cycle. 



® 
chloroplast stroma . / -st atp 



ferredoxin-NADP reductase 



light 




Thylakoid Lumen 



Figure 4: The light reactions includes the movement of electrons down the electric transport chain, splitting 
water and releasing hydrogen ions into the thylakoid space. 



78 



(Source: http://commons.wikimedia.0rg/wiki/lmage:Thylakoid_membrane.png, License: Public Domain) 

Stage 2: Producing Food 

During the Calvin cycle, which occurs in the stroma of the chloroplast, glucose is formed from carbon dioxide 
and the products of the light reactions. During the first step C0 2 is attached to a 5-carbon molecule (called 

Ribulose-5-Phosphate, RuBP), forming a 6-carbon molecule. This reaction is catalyzed by an enzyme named 
RuBisCo, which is the most abundant protein in plants and maybe on Earth! The 6-carbon molecule formed 
by this reaction immediately splits into two 3-carbon molecules, and the 3-carbon molecule is rearranged 
to a 3-carbon carbohydrate. The energy and electrons needed for this process are provided by the ATP and 
NADPH produced earlier in photosynthesis. The "food" made by photosynthesis is formed from the 3-carbon 
carbohydrate. Two 3-carbon carbohydrates combine to form glucose, a 6-carbon carbohydrate. Next, the 
6-carbon RuBP must be reproduced so the Calvin cycle can start again. 




Figure 5: The Calvin Cycle begins with carbon fixation, or carbon dioxide attaching to the 5-carbon molecule 
RuBP, forming a 6-carbon molecule and splitting immediately in to two 3-carbon molecules. This is shown 
at the top of the figure. This carbon molecule is then reduced to a 3-carbon carbohydrate, shown at the 
bottom of the figure. The energy and reducing power needed for this process are provided by the ATP and 
NADPH produced from the light reactions. Next , RuBP must be reproduced so the Calvin cycle can continue. 
The carbons are the small black circles. You can keep track of the number of carbons at each stage by 
counting these circles. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Calvin-cycle3.png, License: CC-SA) 



79 



The 3-carbon product of the Calvin cycle can be converted into many types of organic molecules. Glucose, 
the energy source of plants and animals, is only one possible product of photosynthesis. Glucose is formed 
by two turns of the Calvin cycle. Glucose can be formed into long chains as cellulose, a structural carbohy- 
drate, or starch, a long-term storage carbohydrate. The product of the Calvin cycle can also be used as the 
backbone of fatty acids, or amino acids, which make up proteins. 

Photosynthesis is crucial to most ecosystems since animals obtain energy by eating other animals, or plants 
and seeds that contain these organic molecules. In fact, it is the process of photosynthesis that supplies 
almost all the energy to an ecosystem. 

Lesson Summary 

• The net reaction for photosynthesis is that carbon dioxide and water, together with energy from the sun, 
produce glucose and oxygen. 

• During the light reactions of photosynthesis, solar energy is converted into the chemical energy of ATP 
and NADPH. 

• During the Calvin cycle, the chemical energy of ATP and NADPH is used to convert carbon dioxide into 
glucose. 

Review Questions 

1 . What is the energy-capturing stage of photosynthesis? (Beginning) 

2. What is the product of the light reactions? (Intermediate) 

3. What are the ATP and NADPH from the light reactions used for? (Challenging) 

4. Where does the oxygen released by photosynthesis come from? (Intermediate) 

5. What happens to the glucose produced from photosynthesis? (Challenging) 

6. Describe the structures of the chloroplast where photosynthesis takes place. (Challenging) 

7. What is the significance of the electron transport chain? (Challenging) 

8. What are the reactants required for photosynthesis? (Beginning) 

9. What are the products of photosynthesis? (Beginning) 

Further Reading I Supplemental Links 

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html 

http://photoscience.la.asu.edu/photosyn/education/photointro.html 

http://www.pbs.Org/wgbh/nova/methuselah/photosynthesis.html# 

http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html 

http://www.biology4all.com/resources_library/details. asp?ResourcelD=43 

http://www.enwikipedia.org/ 

Vocabulary 

ATP synthase ^ n enz y me that uses the energy of the movement of H + ions to make ATP. 

Calvin cycle The reactions of photosynthesis in which carbon dioxide is converted into glu- 

cose, which is a type of sugar; also known as the light independent reactions. 



80 



chlorophyll Green pigment in leaves; helps to capture solar energy. 

chloroplast The organelle in which photosynthesis takes place. 

cyanobacteria Photosynthetic bacteria; also known as blue-green bacteria, or blue-green algae. 

electron transport chain A series of electron carrier molecules that transfers electrons. 

light reactions The reactions of photosynthesis that only occur during daylight hours in which 

the energy of sunlight is captured; also known as the light-dependent reactions. 

NADPH A high energy electron carrier produced during the light reactions; carries the 

energy in the electrons to the Calvin cycle. 

photosynthesis The process by which plants use the sun's energy to make their own "food" from 

carbon dioxide and water; process that converts the energy of the sun, or solar 
energy, into carbohydrates, a type of chemical energy. 

stomata Special pores in leaves; carbon dioxide enters the leaf and oxygen exits the 

leaf through these pores. 

stroma Fluid in the chloroplast interior space; surrounds the thylakoids. 

thylakoid Flattened sacs within the chloroplast; formed by the inner membranes. 

Review Answers 

1. The light reactions 

2. ATP and NADPH 

3. The Calvin cycle, to reduce carbon dioxide to carbohydrate. 

4. Splitting water 

5. Can be used in cellular respiration and to make starch, cellulose, lipids, proteins, etc 

6. Thylakoid membranes- light reactions; Stroma- Calvin cycle 

The energy obtained allows carriers to create a H + gradient, which helps generate ATP. 

8. Carbon dioxide, water 

9. Glucose, oxygen 

Points to Consider 

• How is glucose turned into an usable form of energy called ATP? 

• How do you gain energy from the food you eat? 

• What would provide more energy- a bowl of pasta or a small piece of candy? 

• What "waste" gas do you exhale? 

Cellular Respiration 

Lesson Objectives 

• Write and explain the chemical formula for cellular respiration. 

• Explain the two states of cellular respiration. 

• Compare photosynthesis with cellular respiration. 



81 



• Describe the results of fermentation and understand when fermentation is needed. 

Check Your Understanding 

• Where does the energy captured at the beginning of photosynthesis originate from? 

• What is the form of chemical energy produced by photosynthesis? 

• What occurs in oxidation and reduction reactions? 

Introduction 

How does the food you eat provide energy? When you need a quick boost of energy, you might reach for 
an apple or a candy bar. Although foods with sugars can give you a quick boost of energy, they cannot be 
used for energy directly by your cells. Energy is simply stored in these foods. Through the process of cellular 
respiration, the energy in food is converted into energy that can be used by the body's cells. In other words, 
glucose (and oxygen) is converted into ATP (and carbon dioxide and water). ATP is the molecule that provides 
energy for your cells to perform work, such as contracting your muscles as you walk down the street or 
performing active transport. Cellular respiration is simply a process that converts one type of chemical energy, 
the energy stored in sugar, into another type, ATP. 

Overview of Cellular Respiration 

Most often, cellular respiration proceeds by breaking down glucose into carbon dioxide and water. As this 
breakdown of glucose occurs, energy is released. The process of cellular respiration includes the conversion 
of this energy into ATP. The overall reaction for cellular respiration is as follows: 

C 6 H 12 6 + 60 2 — * 6C0 2 + 6H 2 

Notice that the equation for cellular respiration is the direct opposite of photosynthesis. While water was 
broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen 
to form water. While photosynthesis requires carbon dioxide and releases oxygen, cellular respiration requires 
oxygen and releases carbon dioxide. This exchange of carbon dioxide and oxygen in all the organisms that 
use photosynthesis and/or cellular respiration worldwide, helps to keep atmospheric oxygen and carbon 
dioxide at somewhat stable levels. 

Cellular respiration doesn't happen all at once, however. Glucose is broken down slowly so that cells convert 
as much sugar as possible into the usable form of energy, ATP. Still, some energy is lost in the process in 
the form of heat. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 
38 molecules of ATP. Although the process is not 100% efficient, it is much more efficient than a car engine 
obtaining energy from gasoline. 

Cellular respiration can be divided into three phases. 

1. Glycolysis: the breakdown of glucose. 

2. The citric acid cycle: the formation of electron carriers. 

3. The electron transport chain: the formation of ATP. 

In eukaryotic cells, the first phase takes place in the cytoplasm of the cell, while the other phases are carried 
out in the mitochondria. This organelle is known as the "powerhouse" of the cell because this is the organelle 
where the ATP that powers the cell is produced. 

Glycolysis 

The first step of cellular respiration is glycolysis. During glycolysis, the 6-carbon glucose is practically "cut 
in half, broken down into two 3-carbon pyruvate molecules. Glycolysis requires an initial energy-investment 
step, although in the end, glycolysis produces more energy than was initially invested. Two ATP molecules 



82 



are used to convert glucose into the two 3-carbon pyruvate molecules. These 3-carbon molecules are then 
oxidized, which means that they lose electrons, as electrons are transferred to the high energy electron 

acceptor NAD + , producing the electron carrier NADH. This oxidation step helps produce 4 ATP molecules 
from ADP. That means, taking into account the initial investment of 2 ATP molecules, glycolysis has a net 
production of 2 ATP. 

Table 1 : An Overview of Glycolysis 



Inputs 


Outputs 


Glucose (6-carbon 
molecule) 


2 pyruvate (3-car- 
bon molecule) 


2NAD + 


2 NADH (electron 
carrier) 


2 ATP (energy) 


2 ADP 


4 ADP 


4 ATP (energy) 



After glycolysis, the pyruvate can go down several different paths. If there is oxygen available, the pyruvate 
moves inside the mitochondrion to produce more ATP during further break-down stages. In the absence of 
oxygen, the fermentation process begins. 

Inside the Mitochondria 

If oxygen is available, the next step of cellular respiration is moving the pyruvate into the mitochondria. The 
mitochondria have a double membrane. The inner membrane is known as the cristae, and is folded to form 
many internal layers. Some steps of cellular respiration occur in the cristae, while others take place in the 
matrix, the inner compartment of the mitochondrion that is filled with enzymes in a gel-like fluid. 



ATP synthase particles 



Matrix 



cristae 




Inner membrane 
Outer membrane 



Figure 1: Most of the reactions of cellular respiration are carried out in the mitochondria. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Diagram_of_an_animal_mitochondrion.svg, License: 
Public Domain) 

Within the mitochondria the Kreb's cycle or citric acid cycle occurs. The citric acid cycle is a series of 
oxidation steps that produce NADH and FADH 2 , another type of electron carrier. These electron carriers will 

be used in the final step of cellular respiration. To begin the Kreb's cycle, the 3-carbon pyruvate from glycol- 



83 



ysis must be converted into a 2-carbon molecule, which then can enter the cycle. During the cycle carbon 
dioxide is produced. Two molecules of ATP are also produced per each initial glucose molecule. 

Table 2: An Overview of the Citric Acid Cycle 



Inputs 


Outputs 


2 two-carbon molecules 


4C0 2 


6NAD + 


6 NADH (electron carrier) 


2FAD + 


2 FADH 2 (electron carrier) 


2ADP 


2 ATP (energy) 



In the final steps of cellular respiration, the electron transport chain accepts the electrons from glucose 
that are being carried by NADH and FADH 2 . These electrons are passed along the chain until they are finally 

combined with oxygen, which with the addition of hydrogen ions, becomes water. That is the key reason 
why this process only occurs in the presence of oxygen. 

As the electrons move down the electron transport chain, energy is released and later used to synthesize 
ATP. The process of ATP synthesis is exactly the same as photosynthesis; hydrogen ions are pumped 
across the cristae of the mitochondira, forming a chemiosmotic gradient, and ATP synthase uses the energy 
of the movement of hydrogen ions back across the membrane, from high to low concentration, to make 
ATP. 

Because oxygen is the final electron acceptor in this process, the electron transport chain can only occur 
in the presence of oxygen. This is known as aerobic respiration. However, there is not always enough 
oxygen present for aerobic respiration to occur. In this case, the next step after glycolysis will be fermentation 
instead of the citric acid cycle. 



Mitochondrial Electron Transport Chain 




Figure 2: During electron transport, electrons from glucose (carried by NADH and FADH2) are passed along 
until they are finally combined with oxygen, which with the addition of hydrogen ions, becomes water. 



84 



Meanwhile, hydrogen ions are pumped across the cristae of the mitochondria, forming a gradient, and ATP 
synthase uses the energy of the movement of hydrogen ions back across the membrane, from high to low 
concentration, to make ATP. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Etc2.png, License: GNU-FD) 

Fermentation 

Sometimes cellular respiration is anaerobic, occurring in the absence of oxygen. In the process of fermen- 
tation, the NAD + is recycled so that is can be reused in the glycolysis process. No additional ATP is produced 
during fermentation, so the organism only obtains the two net ATP molecules per glucose from glycolysis. 

Yeasts (single-celled eukaryotic organisms) carry on alcoholic fermentation in the absence of oxygen, 
making ethyl alcohol (drinking alcohol) and carbon dioxide. Alcoholic fermentation is central to bread baking. 
The carbon dioxide bubbles allow the bread to rise, and the alcohol evaporates. In wine making, the sugars 
of grapes are fermented to produce the wine. 

Animals and some bacteria and fungi carry out lactic acid fermentation. Lactate (lactic acid) is a waste 
product of this process. Our muscles undergo lactic acid fermentation during strenuous exercise, when 
oxygen cannot be delivered to the muscles quickly enough. The buildup of lactate is what makes your 
muscles sore after vigorous exercise. Bacteria that produce lactate are used to make cheese and yogurt. 
Tooth decay is also accelerated by lactate from the bacteria that use the sugars in your mouth. In all these 

types of fermentation, the goal is the same: to recycle NAD + for glycolysis. 




Figure 3: Products of fermentation include cheese (lactic acid fermentation) and wine (alcoholic fermentation). 
(Source: http://www.flickr.com/photos/gareandkitty/244555951/, License: CC-SA) 

Lesson Summary 

• Cellular respiration is the breakdown of glucose to release energy in the form of ATP. 

• Glycolysis, the conversion of glucose into two 3-carbon pyruvate molecules, is the first step of cellular 
respiration. 

• If oxygen is available, the pyruvate enters the mitochondria and goes through a series of reactions, in- 
cluding the citric acid cycle, to produce more ATP. 



85 



• If oxygen is not available, the pyruvate is reduced during the process of fermentation to free up more 
NAD + for glycolysis, and there is no net gain of ATP. 

Review Questions 

1 . What are the products of alcoholic fermentation? (Beginning) 

2. What is the metabolic process where glucose is ultimately converted to two molecules of pyruvate? (In- 
termediate) 

3. Why do your muscles get sore after vigorous exercise? (Beginning) 

4. What is the purpose of fermentation? (Beginning) 

5. Where does the citric acid cycle take place? (Beginning) 

6. Write the chemical reaction for the overall process of cellular respiration. (Challenging) 

7. Which is more efficient, aerobic or anaerobic cellular respiration? (Intermediate) 

8. What are the important electron-accepting enzymes in cellular respiration? (Challenging) 

9. What is chemiosmosis? (Challenging) 

Further Reading I Supplemental Links 

http://en.wikipedia.org/wiki/Cellular_respiration 

http://biology.clc.uc.edu/Courses/bio104/cellresp.htm 

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookGlyc.html 

http://biology.clc.uc.edu/Courses/bio104/cellresp.htm 

http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html 

Vocabulary 

aerobic respiration Cellular respiration in the presence of oxygen. 

alcoholic fermentation Fermentation in the absence of oxygen; produces ethyl alcohol (drinking alcohol) 

and carbon dioxide; occurs in yeasts. 

anaerobic respiration Cellular respiration in the absence of oxygen; fermentation. 

ATP A useable form of energy inside the cell; adenosine triphosphate. 

cellular respiration The process in which the energy in food is converted into energy that can be 

used by the body's cells; in other words, glucose (and oxygen) is converted into 
ATP (and carbon dioxide and water). 

citric acid cycle Middle phase of cellular respiration; formation of electron carriers occurs during 

this phase; also known as the Kreb's cycle. 

cristae The inner membrane of the mitochondria. The inner membrane of the mitochon- 

dria. 

electron transport chain Last phase of cellular respiration; used to power the formation of ATP occurs 

during this phase. 

FADH 2 Electron carrier produced during the Kreb's cycle. 

fermentation Anaerobic respiration in which NAD + is recycled so that is can be reused in the 

glycolysis process. 



86 



glycolysis First phase of cellular respiration; breakdown of glucose occurs during glycolysis; 

produces two 3-carbon pyruvate molecules. 

lactic acid fermentation Anaerobic respiration that recycles NAD + for glycolysis; occurs in animals and 

some bacteria and fungi. 

matrix The inner compartment of the mitochondrion that is filled with enzymes in a gel- 

like fluid. 

mitochondria Organelle where cellular respiration occurs; known as the "powerhouse" of the 

cell because this is the organelle where the ATP that powers the cell is produced. 

NADH Electron carrier produced during glycolysis and the citric acid cycle. 

Review Answers 

1. ethanol, carbon dioxide 

2. glycolysis 

3. accumulation of lactate from lactic acid fermentation 

to free up NAD + so glycolysis can continue 

5. in the cytoplasm 

6. in the mitochondria 

7. glucose + oxygen -> carbon dioxide, water, and energy 

8. aerobic 

9 - NAD + and FAD 

1Q The process where ATP is synthesized as hydrogen ions flow down a gradient. 

Points to Consider 

• Now that we know how the cell gets its energy, we are going to turn our attention to cell division. Cell 
division is a highly regulated process. 

• What do you think could happen if your cells divide uncontrollably? 

• When new life is formed, do you think it receives all the DNA of the mother and the father? 

• Why do you think you might need new cells throughout your life? 



87 



88 



5. Cell Division, Reproduction, and DNA 



Cell Division 

Lesson Objectives 

• Explain why cells need to divide. 

• List the stages of the cell cycle and explain what happens at each stage. 

• List the stages of mitosis and explain what happens at each stage. 

Check Your Understanding 

• What is the cell theory? 

• In what part of your cells is the genetic information located? 

Introduction 

Imagine the first stages of a life. In humans, a sperm fertilizes an egg, forming the first cell. From that one 
cell, an entire baby with trillions of cells will develop. How does a new life go from one cell to so many? The 
cell divides in half, creating two cells. Then those two cells divide. The new cells continue to divide and divide. 
One cell becomes two, then four, then eight, and soon (Figure 1). Rapid cell division allows the development 
of new life, but cell division must be tightly regulated. If the body's close regulation of cell division is disrupted 
later in life, diseases such as cancer can develop. Cancer involves cells that divide in an uncontrolled 
manner. Therefore, much research into cell division is underway across the globe in effort to further understand 
this process and find a cure for cancer. 




Figure 1: Cells divide repeatedly to produce an embryo. Previously the one-celled zygote divided to make 
two cells (a). Each of the two cells divides to yield four cells (b), then the four cells divide to make eight 
cells(c), and so on. Through cell division, an entire embryo forms from one initial cell. 



89 



(Source: http://commons.wikimedia.Org/wiki/lmage:Gray9.png, License: Public Domain) 

Why Cells Divide 

Besides the development of a fetus, there are many other reasons that cell division is necessary to life. To 
grow and develop, you must form new cells. Imagine how often your cells must divide during a growth spurt. 
Growing just an inch requires countless cell divisions. 

Another reason for cell division is to repair damaged cells. Imagine you cut your finger. After the scab forms, 
it will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come 
from? Remember that according to the cell theory, all cells must come from pre-existing cells. In order to 
make new skin cells, some of your existing skin cells had to undergo cell division. 

Besides suffering physical damage, your cells can simply wear out. Over time you must replace old and 
worn-out cells. Again, cell division is essential to this process. You can only make new cells by dividing 
similar pre-existing cells. 

The Cell Cycle 

The process of cell division in eukaryotic cells is carefully regulated. The cell cycle which in essence is the 
lifecycle of a cell, is composed of a series of steps that lead to cell division (Figure 2). These steps can be 
divided into two main components: interphase and mitosis. Interphase is when the cell mainly performs its 
"everyday" functions; for example, it is when a kidney cell does what a kidney cell is supposed to do. On 
the other hand, mitosis is when the cell prepares to become two cells. Some cells, like nerve cells, do not 
complete the cell cycle and divide, while others divide repeatedly 

Most of the cell cycle consists of interphase, the time between cell divisions. During this time the cell carries 
out its normal functions and prepares for the next stage. Interphase can be divided into three stages: the 
first growth phase (G1 ), the synthesis phase (S), and the second growth phase (G2). During the G1 stage, 
the cell doubles in size and doubles the number of organelles. Next, during the S stage, the DNA is replicated. 
In other words, an identical copy of all the cell's DNA is made. This ensures that each new cell that results 
after cell division has a set of genetic material identical to that of the parental cell. DNA replication will be 
further discussed in lesson 3. Finally, in the G2 stage proteins are synthesized that will aid in cell division. 
In the end of interphase, the cell is ready to enter the mitotic phase. 




Figure 2: The cell cycle is the repeated process of growth and division. Notice that most of the cell cycle is 
spent in interphase (G1, S, and G2) (I). 

(Source: http://commons.wikimedia.Org/wiki/lmage:Cell_cycle.png, License: GNU-FD) 

During the mitotic phase, nuclear division occurs, which is known as mitosis. Also cytokinesis, the division 
of the cytoplasm, occurs. After cytokinesis, cell division is complete and two genetically identical daughter 



90 



cells have been produced from one parent cell. The term "genetically identical" refers to the fact that each 
resulting cell has an identical set of DNA, and this DNA is also identical to that of the parent cell. 

Mitosis and Chromosomes 

During cell division, two nuclei must form during the process of mitosis, so that one nucleus can be given 
to each of cells that form from cytokinesis. In the nucleus, the genetic information of the cell, DNA, is stored. 
The copied DNA needs to be moved into a new nucleus for the new cell to have a correct set of genetic in- 
structions. 

The DNA in the nucleus is condensed into chromosomes, structures composed of DNA wrapped around 
proteins. Each organism has a unique number of chromosomes; in human cells our DNA is divided up into 
23 pairs of chromosomes. When a cell is not undergoing division, such as during interphase, the complex 
of DNA and proteins is a tangled mass of threads known as chromatin. As mitosis begins, however, the 
DNA becomes tightly coiled into the chromosomes which become visible under a microscope. 




Figure 3: The DNA double helix wraps around histone proteins (2) and tightly coils a number of times to 
form a condensed chromosome (5). The chromosomes contains millions of nucleotide bases. This figure 
illustrates the complexity of the coiling process. The red dot shows the location of the centromere, where 
the microtubules attach during mitosis and meiosis. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Chromatin_chromosome.png, License: GNU-FD) 

As mentioned previously, the DNA is replicated during the S stage of interphase. Each chromosome now 
has two identical molecules of DNA, called sister chromatids, forming the "X" shaped molecule depicted 
in Figure 3. During mitosis, the two sister chromatids must be split apart to give rise to two identical chromo- 
somes (in essence, each resulting chromosome is made of 1/2 of the "X"). Through this process, each 
daughter cell receives one copy of each chromosome. 

Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. During prophase, the 
chromosomes become tightly wound and become visible under the microscope. Also, the nuclear envelop 
dissolves, and the spindle begins to form. The spindle is a structure containing many fibers that helps to 
move the chromosomes. By late prophase, the chromosomes are attached to the spindle fibers. The spindle 
fibers will later pull the chromosomes into alignment. 

During metaphase, the chromosomes are now lined up across the center of the cell. The chromosomes line 
up in a row, one on top of the next. During anaphase, the two sister chromosomes of each chromosome 
separate, resulting in two sets of identical chromosomes. During telophase, the spindle dissolves and nuclear 
envelopes form around the chromosomes. Each new nucleus contains the exact same number and types 
of chromosomes as the original cell. The cell is now ready for cytokinesis, producing two genetically identical 
cells, each with its own nucleus. 



91 



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IV 




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Figure 4: An overview of mitosis: during prophase (I and II) the chromosomes condense, during metaphase 
the chromosomes line up (III and IV), during anaphase the sister chromatids are pulled to opposite sides of 
the cell (V and VI), during telophase the nuclear envelope forms (and VII and VIM). 

(Source: http://commons.wikimedia.Org/wiki/lmage:Gray2.png, License: Public Domain) 




Figure 5: This is a picture of dividing plant cells. Cell division in plant cells differs slightly from animal cells 
as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different 



92 



stages of mitosis? 

(Source: http://commons.wikimedia.Org/wiki/lmage:Wilson1900Fig2.jpg, License: Public Domain) 

Lesson Summary 

• Cells divide for growth, development, reproduction and replacement of injured or worn-out cells. 

• The cell cycle is a serious of regulated steps by which a cell divides. 

• During mitosis, the newly duplicated chromosomes are divided into two daughter nuclei. 

Review Questions 

1. In what phase of mitosis are chromosomes moving toward opposite sides of the cell? (Beginning) 

2. In what phase of mitosis are the duplicated chromosomes first condensing? (Beginning) 

3. What step of the cell cycle is the longest? (Beginning) 

4. What is the term for the division of the cytoplasm? (Intermediate) 

5. What happens during the S stage of interphase? (Beginning) 

6. Interphase used to be considered the "resting" stage of the cell cycle. Why is this not correct? (Challenging) 

7. What are some reasons that cells divide? (Challenging) 

8. During what stage of the cell cycle does the cell double in size? (Intermediate) 

9. Why must cell division be tightly regulated? (Intermediate) 
1Q What is the purpose of mitosis? (Challenging) 

Further Reading I Supplemental Links 

http://en.wikipedia.org/wiki/Mitosis 

http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/cells3.html 

http://biology.clc.uc.edu/courses/bio104/mitosis.htm 

http://en.wikipedia.org/wiki/Cell_cycle 

http://www.cellsalive.com/mitosis.htm 

http://www.wisc-online.com/objects/index_tj .asp?objlD=AP1 3604 

Vocabulary 

anaphase Third phase of mitosis where sister chromatids separate and move to opposite sides of 

the cell. 

cell cycle Sequence of steps in eukaryotic cells that leads to cell division. 

chromatin Complex of DNA and proteins that is visible when a cell is not dividing. 

chromosomes DNA wound around proteins; forms during prophase of mitosis and meiosis. 

cytokinesis Division of the cytoplasm after mitosis or meiosis. 

interphase Stage of the cell cycle when DNA is synthesized and the cell grows; composed of the first 
three phases of the cell cycle. 

metaphase Second phase of meiosis where the chromosomes are aligned in the center of the cell. 



93 



mitosis Sequence of steps in which a nucleus is divided into two daughter nuclei, each with an 

identical set of chromosomes. 

prophase Initial phase of mitosis where chromosomes condense, the nuclear envelope dissolves 

and the spindle begins to form. 

spindle Fibers that move chromosomes during mitosis and meiosis. 

telophase Final phase of mitosis where a nuclear envelop forms around each of the two sets of 

chromosomes. 

Review Answers 

1. anaphase 

2. prophase 

3. interphase 

4. cytokinesis 

5. The DNA is replicated (copied). 

6. A lot is happening during interphase: DNA is replicated, organelles double, cell grows, etc. 

7. Development and growth, and to replace worn-out or injured cells. 

8. G1 of interphase 

9. Unregulated cell division can lead to cancer. 

1Q During cell division, newly duplicated chromosomes in one nucleus separate to form two new nuclei. 

Points to Consider 

• How might a cell without a nucleus divide? 

• How are new cells made that incorporate the DNA of two parents? 

Reproduction 

Lesson Objectives 

• Name the types of asexual reproduction. 

• Explain the advantage of sexual reproduction. 

• List the stages of meiosis and explain what happens in each stage. 

Check Your Understanding 

• Can something that does not reproduce still be considered living? 

• What stores the genetic information that is passed on to offspring? 

• How many chromosomes are in the human nucleus? 



94 



Introduction 

Can an organism be considered alive if it cannot make the next generation? For a species to survive, repro- 
duction, the ability to make the next generation, is absolutely necessary. For a species to be successful, it 
not only needs to be well adapted to its environment, but also needs to be successful at reproduction. Re- 
production allows a population of organisms to pass on their genetic information to the next generation. 
There are many different ways that organisms reproduce, and these methods are categorized as either 
sexual or asexual reproduction. There are advantages and disadvantages to each method, but the result is 
always the same: a new life begins. 

Asexual Reproduction 

Some organisms can reproduce asexually, meaning that the offspring have a single parent and share the 
exact same genetic material as the parent. The advantage of asexual reproduction is that it can be very 
quick and does not require the meeting of two individuals of the opposite sexes. The disadvantage of 
asexual reproduction is that it does not involve genetic recombination, a process that can result in an 
adaptive new set of traits. For example, you might inherit one advantageous trait from your maternal 
grandmother, another adaptive trait from your paternal grandmother, and other adaptive traits from your 
paternal grandfather. You have the benefit of the many genes from two lineages combining in a new way. 
An organism that is born through asexual reproduction, however, only has the DNA from one parent, and it 
is the exact copy of that parent. Therefore, no new combinations of traits can happen. 

Prokaryotic organisms, which as you might recall are single-celled, reproduce asexually. Bacteria reproduce 
through binary fission, where they basically divide in half (Figure 1). First, their chromosome replicates 
and the cell enlarges. After cell division, the two new cells each have one identical chromosome. Mitosis is 
not necessary as there is no nucleus. Then new membranes form to separate the two cells. This simple 
process is beneficial to the bacteria, allowing very rapid reproduction. 



© 



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Figure 1: Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two 
bacteria. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Bacilli_division_diagram.png, License: GNU-FD) 

There are also several animals that can reproduce asexually. Flatworms can divide in two, then each half 
regenerates into a new flatworm identical to the original. Many types of insects, fish, and lizards can reproduce 
asexually through parthenogenesis. Parthenogenesis is a process by which an unfertilized egg cell grows 
into a new organism. The resulting organism has half the amount of genetic material of the parent, as the 
starting egg cell has half the amount of DNA compared to the parent. Parthenogenesis is common in hon- 
eybees. The fertilized eggs in a hive become workers, while the unfertilized eggs become drones. 



95 



Egg cells (and also sperm cells) are produced through a cell division mecha- 
nism in which the amount of DNA is halved. This process is called meiosis 
and will be discussed shortly. 




Figure 2: This Komodo dragon was born by parthenogenesis. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Parthkomodo.jpg, License: CC-SA) 

Sexual Reproduction 

During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a sep- 
arate male and female sex, with the male producing sperm and the female producing eggs. When a sperm 
and egg meet, a zygote, the first cell of a new organism, is formed. The zygote will divide and grow into the 
embryo. 







Figure 3: During sexual reproduction, a sperm fertilizes an egg. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Sperm-egg.jpg, Permissions: Public Domain) 

Animals often have gonads, specialized organs that produce eggs or sperm. The male gonads are the 
testes, which produce the sperm, and the female gonads are the ovaries, which produce the eggs. Sperm 
and egg, the two sex cells, are known as gametes, and unite through a variety of methods. Fish and other 



96 



aquatic animals release their gametes in the water, which is called external fertilization. Animals that live 
on land, however, usually practice internal fertilization. Typically males have a penis that deposits sperm 
into the vagina of the female. Other anatomical features can accomplish the same goal; birds, for example, 
have a chamber called the cloaca that they place close to another bird's cloaca to deposit sperm. Whatever 
method of fertilization is used, the net result is the same: a zygote that contains DNA from both the male 
and female. 




Figure 4: This fish guards her eggs, which will be fertilized externally. 

(Source: http://www.flickr.com/photos/chikawatanabe/1888284871/, License: Public Domain) 

Plants also can reproduce sexually, but their reproductive organs are somewhat different than animals' go- 
nads. Most plants are flowering plants, meaning their reproductive parts are contained in a flower. The sperm 
is contained in the pollen, while the egg is contained in the ovary deep within the flower. The sperm can 
reach the egg through several methods. In self-pollination, the egg is fertilized by the pollen of same flower. 
In cross-pollination, the sperm from the pollen from one flower fertilizes the egg of another flower. Cross- 
pollination increases the genetic diversity of the population. Like other types of sexual reproduction, cross- 
pollination allows new combinations of traits. Cross-pollination can be achieved when pollen is carried by 
the wind to another flower, or many flowers rely on animal pollinators, like honeybees, to carry the pollen 
from flower to flower. 




Figure 5: Butterflies receive nectar when they deposit pollen into flowers, resulting in cross-pollination. 
(Source: http://www.flickr.com/photos/emeryjl/456175019/, License: Public Domain) 



97 



Fungi can also reproduce sexually, but instead of female and male sexes, they have (+) and (-) strains. 
When the filaments of a (+) and (-) fungi meet, the zygote is formed. As with the sexual reproduction in 
plants and animals, each zygote receives DNAfrom two parent strains. 

Meiosis and Gametes 

The formation of gametes, the reproductive cells such as sperm and egg, is necessary for sexual reproduction. 
As gametes are produced, the number of chromosomes must be reduced to half. In humans, our cells have 
23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromosome. For 
each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome 
from your mother. The homologous chromosomes have the same genes, although there might be alternate 
forms of each gene, called alleles, which vary between the chromosomes. These homologous chromosomes 
are separated during gamete formation, therefore gametes have only 23 chromosomes, not 23 pairs. A cell 
with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. 
A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. So when a haploid 
sperm and a haploid egg combine, a diploid zygote will be formed. The process of cell division that reduces 
the chromosome number by half is called meiosis. 

Prior to meiosis, DNA replication occurs, so each chromosome contains two sister chromatids that are 
identical to the original chromosome. Meiosis is divided into two nuclear divisions: meiosis I and meiosis II. 
Each of these nuclear divisions shares many aspects of mitosis and can be divided into the same phases: 
prophase, metaphase, anaphase, and telophase; however, between the two divisions, DNA replication does 
not occur. Through this process, one diploid cell will divide into four haploid cells. 

During meiosis I, the pairs of homologous chromosomes are separated from each other. During prophase 
I, the homologous chromosomes line up together. During this time, crossing-over can occur, the exchange 
of DNA between homologous chromosomes. Crossing-over increases the new allele combinations in the 
gametes. Without crossing-over, the offspring would always inherit all of the many alleles on one of the ho- 
mologous chromosomes. Because of crossing over, the alleles on the homologous chromosomes can be 
scrambled to pass on unique combinations of alleles on the chromosome. Also during prophase I, the 
spindle forms and the chromosomes condense as they coil up tightly. The spindle has the same function 
as in mitosis. 



H* 


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R, 







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Fig. 65. Scheme to illustrate double crossing over. 



Figure 6: During crossing-over, segments of DNA are exchanged between sister chromatids. Notice how 
this can result in an allele (M) on one sister chromatid being moved onto the other sister chromatid. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Morgan_crossover_2.jpg, License: Public Domain) 

During metaphase I, the homologous chromosomes line up in pairs in the middle of the cell; that is, both 
chromosome of a pair will line up together. The maternal chromosomes or paternal chromosomes can each 
attach to either side of the spindle. The assignment of which side is random, so all the maternal or paternal 
chromosomes do not end up in one gamete. The gamete will contain some chromosomes from the mother 



98 



and some chromosomes from the father. Note this is different than during metaphase of mitosis; although 
chromosomes still line up during mitosis, the sister chromatids are separated, and each cell obtains both 
the maternal and paternal chromosome of each pair. 

During anaphase I, the homologous chromosomes separate. In telophase I, the spindle dissolves, but a 
new nuclear envelop does not need to form. That's because after a brief resting stage, the nucleus will divide 
again. No DNA replication happens between meiosis I and meiosis II as the chromosomes are already du- 
plicated, carrying sister chromatids. 

During meiosis II, the sister chromosomes are separated and the gametes are generated. During prophase 
II, the chromosomes condense. In metaphase II the chromosomes line up one on top of the next along the 
equator, or middle of the cell. During anaphase II, the sister chromatids separate. After telophase and cy- 
tokinesis, each cell has divided again. Therefore, meiosis results in four cells with half the DNA of the parent 
cell. In our cells, the parent cell has 46 chromosomes, whereas the cells that result from meiosis have 23 
chromosomes. These cells will become gametes. 

[image:Lsc-0502-07a.png|400px] 

Figure 7: An overview of meiosis. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Meiose_%280verzicht%29.png, License: GNUFD) 



Binary fission 




Figure 8: A comparison between binary fission, mitosis, and meiosis. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/f/f9/Three_cell_growth_types.png, License: GNU- 
FD) 

Lesson Summary 

• Organisms can reproduce sexually or asexually. 

• The gametes in sexual reproduction must have half the DNA of the parent. 



99 



• Meiosis is the process of nuclear division to form gametes. 

Review Questions 

1. What is parthogenesis? (Intermediate) 

2. How can organisms reproduce asexually? (Intermediate) 

3. How would sexual reproduction in a lizard be different than a fish? (Challenging) 

4. Are the viable eggs that birds lay need to be fertilized externally? (Challenging) 

5. How do most plants reproduce sexually? (Challenging) 

6. What is the purpose of meiosis? (Challenging) 

7. What is the advantage of sexual reproduction over asexual reproduction? (Challenging) 

8. If an organism has 12 chromosomes in its cells, how many chromosomes will be in its gametes? (Chal- 
lenging) 

9. During what phase of meiosis do homologous chromosomes separate? (Intermediate) 
1Q In what phase of meiosis do homologous chromosomes pair up? (Intermediate) 

Further Reading I Supplemental Links 

• http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html 

• http://www.biology.arizona.edu/Cell_BIO/tutorials/meiosis/page3.html 

• http://www.cellsalive.com/meiosis.htm 

• http://www.yout.ube. com/watch ?v=MqaJqLL49aO&NR=1 

• http://en.wikipedia.org/ 

Vocabulary 

allele An alternative form of a gene. 

asexual reproduction A form of reproduction in which a new individual is created by only one parent. 

binary fission An asexual form of reproduction where a cell splits into two daughter cells. 

crossing-over Exchange of DNA segments between homologous chromosomes; occurs during 

prophase I of meiosis. 

cross-pollination Sexual reproduction in plants where sperm from the pollen of one flower is received 

by the ovary of another flower. 

diploid When a cell has two sets of chromosomes. 

gametes Cells involved in sexual reproduction; typically egg and sperm cells. 

gonads Organ that produces gametes, such as the ovaries and testes. 

haploid When a cell has only one set of chromosomes, typical of a gamete. 

internal fertilization Reproduction occurs through the internal deposit of gametes. 

external fertilization Reproduction where the eggs are fertilized outside the body. 

meiosis Nuclear division that results in haploid gametes. 

ovaries Female gonads in animals that produce eggs. 

parthenogenesis Reproduction where an unfertilized egg develops into a new individual. 

sexual reproduction Reproduction where gametes from two parents combine to make an individual with 
an unique set of genes. 



100 



sister chromatids Two genetically identical chromosome segments that form after DNA replication. 

testes Male gonads in animals that produce sperm. 

zygote Single cell that is formed after the fertilization of an egg; the first cell of a new or- 

ganism. 

Review Answers 

1. When an unfertilized egg grows into a new organism. 

2. parthenogenesis, fission, stem cuttings, etc. 

3. Lizards live on land and must have some type of internal fertilization, while fish have external fertilization. 

4. No, birds have internal fertilization. 

5. The transfer of sperm from the pollen from one flower to another flower's ovary. 

6. To form gametes with half the chromosomes of the parent cell. 

7. Genetic recombination (genetic variation) can result in beneficial (adaptive) new traits. 

8. 6 

9. anaphase of meiosis I 
1Q prophase of meiosis I 

Points to Consider 

• What must be replicated prior to mitosis? 

• How do you think DNA might be replicated? 

• What might happen if there is a mistake during DNA replication? 

DNA, RNA, and Protein Synthesis 

Lesson Objectives 

• Explain the chemical composition of DNA. 

• Explain how DNA synthesis works. 

• Explain how proteins are coded for and synthesized. 

• Describe the three types of RNA and the functions of each. 

Check Your Understanding 

' What is the purpose of DNA? 

• When is DNA replicated? 

Introduction 

Much research in the past fifty years has been focused on understanding the genetic material, DNA. Under- 
standing how DNA works has brought with it many useful technologies. DNA fingerprinting allows police to 
match a criminal to a crime scene. Transgenic crops, or crops that contain altered DNA, have improved 



101 



yields for farmers. And you can now test your DNA to find out the chance that your future children may be 
at risk for a rare genetic disorder. Although we can do some really complicated things with DNA, the chem- 
ical structure of DNA is remarkably simple and elegant. 

What is DNA? 

DNA, is the material that makes up our chromosomes and stores our genetic information. This genetic infor- 
mation is basically a set of instructions that tell your cells what to do. DNA is an abbreviation for deoxyribonu- 
cleic acid. As you may recall, nucleic acids are the class of chemical compounds that store information. The 
deoxyribo part of the name refers to the name of the sugar that is contained in DNA, deoxyribose. 

The chemical composition of DNA is a polymer, or long chain, of nucleotides. Nucleotides are composed 
of a phosphate group, a 5-carbon sugar, and a nitrogen-containing base. The only difference between each 
nucleotide is the identity of the base. There are only four possible bases that make up each DNA nucleotide: 
adenine (A), guanine (G), thymine (T), and cytosine (C). The various sequences of these four bases make 
up the genetic code of your cells. It may seem strange that there are only four letters in the "alphabet" of 
DNA. But since your chromosomes contain millions of nucleotides, there are many, many different combina- 
tions possible with those four letters. 

But how do all these pieces fit together? James Watson and Francis Crick won the Nobel Prize in 1 962 for 
piecing together the structure of DNA. Together with the work of Rosalind Franklin and Maurice Wilkins, 
they determined that the structure of DNA is two strands of nucleotides in a double helix, or a two-stranded 
spiral, with the sugar and phosphate groups on the outside, and the paired bases connecting the two strands 
on the inside of the helix. 




102 



Figure 1: DNA's three-dimensional structure is a double helix. The hydrogen bonds between the bases at 
the center of the helix hold the helix together. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:DNA_Overview.png, License: GNU-FD) 

The bases do not pair randomly, however. When Erwin Chargaff looked closely at the base content in DNA, 
he noticed that the percentage of adenine (A) in the DNA always equaled the percentage of thymine (T), 
and the percentage of guanine (G) always equaled the percentage of cytosine (C). Watson and Crick's 
model explained this result by suggesting that A always pairs with T and G always pairs with C in the DNA 
helix. Therefore A and T, and G and C, are complementary bases. If one DNA strand reads ATGCCAGT, 
the other strand would be made up the complementary bases: TACGGTCA. These base pairing rules state 
that in DNA, A always pairs with T, and G always pairs with C. 



5' End - QH 3' End 

I 3 la 

0=P— C— C 

O C' .C 

I / x o^l 

CH 2 A ' CH 2 



,-o„ y 



I 



V X_r O 



c c 
c— c 

I 

p 



I 

2 0=P— 

I. 

a 



= - c— c 3 

H X ' CH 2 

C C I 

.■ o=p-o 

c— c I . 





i-o- c^ 

i 1 '■ 4 




I 

CH 



A 

\./ J 



3' End 



c 4 x c 1 O 

\ 3 2 „ T . 

C-C 0=F>-0 5 . End 

HO O 



Figure 2: The chemical structure of DNA includes a chain of nucleotides consisting of a 5-carbon sugar, a 
phosphate group, and a nitrogen base. Notice how the sugar and phosphate form the backbone of DNA 
(one strand in blue), with the hydrogen bonds between the bases joining the two strands. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:DNA_Structure.jpg, License: GNU-FD) 

DNA Replication 

The base paring rules are crucial for the process of replication. DNA replication is the process by which 
DNA is copied to form an identical daughter molecule of DNA. During DNA replication, the DNA helix unwinds 
as the weak hydrogen bonds between the paired bases are broken. The two single strands of DNA then 
each serve as a template for a new stand to be synthesized. The new nucleotides are placed in the right 
order because of the base pairing rules. The new set of nucleotides then join together to form a new strand 
of DNA. The process results in two DNA molecules, each with one old strand and one new strand of DNA. 
Therefore, this process is known as semiconservative replication because one strand is conserved in 
each new DNA molecule. 



103 




Figure 3: DNA replication occurs by the DNA strands "unzipping", and the original strands of DNA serve as 
a template for new nucleotides to join and form a new strand. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Dna-split.png, License: Public Domain) 
Protein Synthesis 

The code of DNA, stored in the base sequences, contains the instructions for the order of assembly of amino 
acids to make proteins. Each strand of DNA has many, many separate sequences that code for the production 
of a specific protein. These discrete units of DNA that contain code for the creation of one protein are called 
genes. Proteins are made up of units called amino acids, and the sequence of bases in DNA codes for the 
specific sequence of amino acids in a protein. 

There are about 22,000 genes in every human cell. Does every human cell have the same genes? Yes. 
Does every human cell use the same genes to make the same proteins? No. In a multicellular organism, 
such as us, cells have specific functions because they have different proteins, and they have different proteins 
because different genes are expressed in different cell types. Think of gene expression as if all your genes 
usually are "turned off." Each cell type only "turns on" (or expresses) the genes that have the code for the 
proteins it needs to use. So different cell types "turn on" different genes, allowing different proteins to be 
made, giving different cell types different functions. 

However, DNA does not directly coordinate the production of proteins. Remember that DNA is found in the 
nucleus of the cell, but proteins are made on the ribosomes in the cytoplasm. How do the instructions in the 
DNA get out to the cytoplasm so that proteins can be made? DNA sends out a message, in the form of RNA 
(ribonucleic acid), describing how to make the protein. There are three types of RNA directly involved in 
protein synthesis. Messenger RNA (mRNA) carries the instructions from the nucleus to the cytoplasm. The 
other two forms of RNA, ribosomal RNA (rRNA) and transfer RNA (tRNA) are involved in the process of 
ordering the amino acids to make the protein. This process is called translation and will be discussed below. 
All three RNAs are nucleic acids and are therefore made of nucleotides. The RNA nucleotide is very similar 
to the DNA nucleotide except for the fact that it contains a different kind of sugar, ribose, and the base uracil 
(U) replaces the thymine (T) found in DNA. 



104 



mRNA is created in a method very similar to DNA synthesis. mRNA is also made up of nucleotide units. 
The double helix unwinds and the nucleotides follow basically the same base paring rules to form the correct 
sequence in the mRNA. This time, however, U pairs with each A in the DNA. In this manner, the genetic 
code is securely passed on to the mRNA. The process of constructing a mRNA molecule from DNA is known 
as transcription. 



Gene 



a. Double 
Strand of DNA 



b. Senses 
strand of 
DNA 



c. mRNA 




L 



J L 



Codon 



r 

Codon 



J L 



Codon 



Figure 4: Each gene (a) contains triplets of bases (b) that are transcribed into RNA (c). Every triplet, or 
codon, encodes for a unique amino acid. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Transcripti0n.png, License: Public Domain) 




Figure 5: Base-pairing ensures the accuracy of transcription. Notice how the helix must unwind for transcrip- 
tion to take place. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:DNA_transcripti0n.gif, License: Public Domain) 

The mRNA is directly involved in the protein synthesis process and tells the ribosome how to assemble a 
protein. The base code in the mRNA dictates the order of the amino acids in the protein. But because there 



105 



are only 4 bases in mRNA and 20 different amino acids, one base cannot directly code for one amino acid. 
The genetic code in mRNA is read in "words" of three letters (triplets), called codons. For example, GGU 
encodes for the amino acid glycine, while GUC encodes for valine. This genetic code is universal and used 
by almost all living things. These codons are read in the ribosome, the organelle responsible for protein 
synthesis. In the ribosome, tRNA reads the code and brings a specific amino acid to attach to the growing 
chain of amino acids, which is a protein in the process of being synthesized. Each tRNA carries only one 
type of amino acid and only recognizes one specific codon. The process of reading the mRNA code in the 
ribosome to synthesize a protein is called translation. There are also three codons, UGA, UAA, and UAG, 
which indicate that the protein is complete. They do not have an associated amino acid. As no amino acid 
can be added to the growing polypeptide chain, the protein is complete. 



Amino Acid:- 



Growing 
Protein Chain 



tRNA 




mRNA 



Figure 6: Ribosomes translate RNA into a protein with a specific amino acid sequence. The tRNA binds 
and brings to the ribosome the amino acid encoded by the mRNA. Ribosomes are made of rRNA and proteins. 

(Source: http://commons.wikimedia.Org/wiki/lmage:ProteinTranslation.jpg, License: Public Domain) 




Figure 7: This summary of how genes are expressed shows that DNA is transcribed into RNA, which is 
translated in turn to protein. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Genetic_c0de.svg, License: GNU-FD) 



106 




Figure 8: This chart shows the genetic code used by all organisms. For example, an RNA codon reading 
GUU would encode for a valine (Val) according to this chart. Start at the center for the first base of the three 
base codon, and work your way out. Notice for valine, the second base is a U and the third base of the 
codon may be either a G, C, A, or U. Similarly, glycine (Gly) is encoded by a GGG, GGA, GGC, and GGU. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Codons_aminoacids_table.png, License: GNU-FD) 

Mutations 

The process of DNA replication is not always 1 00% accurate, and sometimes the wrong base is inserted in 
the new strand of DNA. A permanent change in the sequence of DNA is known as a mutation. A mutation 
may have no effect on the phenotype or can cause the protein to be manufactured incorrectly, which can 
affect how well the protein works, or whether it works at all. Usually the loss of a protein function is detrimental 
to the organism. 

However, in rare circumstances, the mutation can be beneficial. For example, suppose a mutation in an 
animal's DNA causes the loss of an enzyme that makes a dark pigment in the animal's skin. If the population 
of animals has moved to a light colored environment, the animals with the mutant gene would have a lighter 
skin color and be better camouflaged. So in this case, the mutation was beneficial. 

There are many possible types of mutations possible in chromosomes. In the case of a point mutation, there 
is a change in a single nucleotide. Other mutations can be more dramatic. A large segment of DNA can be 
deleted, duplicated, inverted, or inserted in the wrong place. These mutations usually result in a non-functional 
protein, or a number of non-functional proteins. A deletion is when a segment of DNA is lost, so there is a 
missing segment in the chromosome. A duplication is when a segment is repeated, creating a longer chro- 
mosome. In an inversion, the segment of DNA is flipped and then reattached to the chromosome. An insertion 
is when a segment of DNA from one chromosome is added to another, unrelated chromosome. 



107 



Types of mutation 
Deletion Duplication Inversion 




Z 

z 
z 

n 

Chromosome 4 



Insertion 



=> 



Chromosome 20 



Chromosome 20 
Chromosome 4 



Translocation 

Chromosome 20 




Derivative 
Chromosome 20 



^> 




Derivative 
Chromosome 4 



Chromosome 4 



Figure 9: Mutations can arise in DNA through deletion, duplication, inversion, insertion, and translocation 
within the chromosome. A deletion is when a segment of DNA is lost from the chromosome. A duplication 
is when a segment is repeated. In an inversion, the segment of DNA is flipped and then re-annealed. An 
insertion or translocation can cause DNA from one chromosome to be added onto another, unrelated chro- 
mosome. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Types-of-mutation.png, License: Public Domain) 

Even if a single base is changed, it can cause a major problem. The substitution of a single base is called 
a point mutation. Sickle cell anemia is an example of a condition caused by a point mutation in the hemoglobin 
gene. In this gene, just the one base change causes a different amino acid to be inserted in the hemoglobin 
protein, causing the protein to fold differently and not function properly in carrying oxygen in the bloodstream. 

If a single base is deleted, it can also have huge effects on the organism because this would cause a 
frameshift mutation. Remember that the bases are read in groups of three by the tRNA. If the reading frame 
gets off by one base, the resulting sequence will consist of an entirely different set of codons. The reading 
of an mRNA is like reading three letter words of a sentence. Imagine you wrote "big dog ate red cat". If you 
take out the second letter, the frame will be shifted so now it will read "bgd oga ter edc at." One single 
deletion makes the whole "sentence", or mRNA, not read correctly. 

Many mutations are not caused by errors in replication. Mutations can happen spontaneously and they can 
be caused by mutagens in the environment. An example of a mutagen is radiation. High levels of radiation 



108 



can alter the structure of DNA. Also, some chemicals, such as those found in tobacco smoke, can be muta- 
gens. Sometimes mutagens can also cause cancer. Tobacco smoke, for example, is often linked to lung 
cancer. 

Lesson Summary 

• DNA stores the genetic information of the cell in the sequence of its 4 bases: adenine, thymine, guanine, 
and cytosine. 

• The information in a small segment of DNA, a gene, is sent by mRNA to the ribosome to synthesize a 
protein. 

• Within the ribosome, tRNA reads the mRNA in sets of three bases (triplets), called codons, which encode 
for the specific amino acids that make up the protein. 

• A mutation is a permanent change in the sequence of bases in DNA. 

Review Questions 

1 . Translate the following segment of DNA into RNA: AGTTC (Intermediate) 

2. Write the complimentary DNA nucleotides to this strand of DNA: GGTCCA (Intermediate) 

3. What makes up a nucleotide? (Intermediate) 

4. Nucleotides are subunits of which polymers? (Intermediate) 

5. Amino acids are subunits that make up what polymer? (Intermediate) 

6. Describe the process of DNA replication (Challenging) 

7. Name a mutagen. (Beginning) 

8. What is made in the process of transcription? (Beginning) 

9. What is made in the process of translation? (Beginning) 
1Q How does RNA encode for proteins? (Challenging) 

Further Reading I Supplemental Links 

http://www.phschool.com/science/biology_place/biocoach/dnarep/intro.html http://nobelprize.org/educa- 
tiona_games/medicine/dna_double_helix/readmore.html http://sickle.bwh.harvard.edu/scd_background.html 
http://www.biostudio.com/demo_freeman_protein_synthesis.htm http://learn.genetics.utah.edu/units/ba- 
sics/transcribe/ http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html 

http://learn.genetics.utah.edu/units/basics/builddna/ http://enwikipedia.org/ 

Vocabulary 

amino acid The units (monomers)that combine to make proteins. 

DNA Deoxyribonucleic acid; a nucleic acid that is the genetic material of all organ- 
isms. 

DNA replication The synthesis of new DNA; occurs during the S phase of the cell cycle. 

double helix Describes the shape of DNA as a double spiral; similar to a spiral staircase. 

gene The inherited unit of DNA that encodes for one protein (or one polypeptide). 

mutagen A chemical or physical agent that can cause changes to accumulate in DNA. 

mutation A change in the nucleotide sequence of DNA. 



109 



nucleotide The units that make up DNA; consists of a 5-carbon sugar, a phosphate 

group, and a nitrogen-containing base 

RNA The nucleic acid that carries the information stored in DNA to the ribosome. 

semiconservative replication Describes how the replication of DNA results in two molecules of DNA, each 

with one original strand and one new strand. 

transcription The synthesis of a RNA that carries the information encoded in the DNA. 

translation The synthesis of proteins as the ribosome reads each codon in RNA, which 

code for a specific amino acid. 

Review Answers 

1. UCAAG 

2. CCAGGT 

3. A sugar, a phosphate group, and a nitrogen-containing base. 

4. DNA and RNA 

5. Proteins 

6. The DNA "unzips" and each strand serves as a template for a new strand of DNA. 

7. Radiation, chemicals in tobacco smoke 

8. RNA 

9. Proteins 

1Q Each set of three RNA nucleotides (codon) codes for a specific amino acid in the protein. 

Points to Consider 

• Your cells have "proofreaders" that replace mismatched pairs that occurred during DNA synthesis. How 
would that affect the rate of mutation in your body? 

• There are many diseases due to mutations in the DNA. These are known as genetic diseases, and many 
can be passed onto the next generation. Think about how a single base change cause a huge medical 
problem like sickle cell anemia. 

• Your DNA contains the instructions to make you. So is everyone's DNA different? Can it be used to 
distinguish individuals, like a fingerprint? 



110 



6. Genetics 



Gregor Mendel and the Foundations of Genetics 

Lesson Objectives 

• Explain Mendel's law of segregation. 

• Draw a Punnett square to make predictions about the traits of the offspring of a simple genetic cross. 

Check Your Understanding 

• What is the genetic material of our cells? 

• How does meiosis affect the chromosome number in gametes? 

introduction 

For centuries people have been fascinated with the inheritance of human traits. People might say, "You 
have your father's eyes." or, "You have red hair like your granddad; it must have skipped a generation." 
These comments show an appreciation of the laws of human inheritance. We inherit traits from our ancestors, 
and sometimes traits can stay hidden and show up in later generations. Genetics, the study of inheritance, 
explains how traits are passed on from one generation to the next. Due to recent developments in the field 
of genetics, we can now seek to understand the inheritance of disease. People today may ask "What are 
the chances my child will have cystic fibrosis?" and "What is the likelihood that I may have breast cancer if 
my grandmother had it?" Genetic counselors are trained to address families' questions about the probabilities 
of passing on a genetic disorder. When genetic counselors sit down with families to discuss these types of 
questions, it's amazing that their answers are derived from the fundamentals of genetics discovered by a 
monk in the 1800s. 




Figure 1 : Gregor Mendel. 



111 



(Source: http://commons.wikimedia.Org/wiki/lmage:Mendel_Gregor_1822-1884.jpg, License: Public Domain) 

Mendel's Experiments 

The laws of heredity were first developed by an Austrian monk, Gregor Mendel, in the 1 800s. To study ge- 
netics, Mendel chose to work with pea plants because they had easily observable traits and a short gener- 
ation time. For example, pea plants are either tall or short, which are easily identifiable traits. Furthermore, 
peas can either self pollinate or be cross-pollinated by hand, by transferring the pollen from one flower to 
the stigma of another. In this way, Mendel could carefully observe the results of crosses between two different 
types of plants. He studied the inheritance patterns for many different traits in peas, including round seeds 
versus wrinkled seeds, white flowers versus purple flowers, and tall plants versus short plants. Because of 
his work, Mendel can be considered the father of genetics. 



Seed 


Form 


Cotyledons 


© 


CO 


Grey & 
Round 


>ellow 


® 


<QG> 


White*. 
Wrinkled 


Green 


1 


2 



Flower 
Color 



Q 



Wh te 



Violet 
3 



Pod 


Form 


Color 


Nfe^ 


Nfc^ 


Full 


"fellow 


Ntes^ 


Nfc^ 


Constricted 


Green 


4 


5 



Stern 

Place Size 





Axial pods, , _, (6 _ 7ft) 



terminal pods,-., ( 1ft , 
Flowers topi Short* -lft) 



Figure 2: The Laws of Heredity. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Mendel_seven_characters.svg, License: Public Domain) 

During Mendel's time, most people believed that traits were contributed from both parents and blended to- 
gether as they were passed down from generation to generation. For example, if you crossed a short plant 
and a tall plant, they would expect the offspring to be medium-sized plants. What Mendel observed, however, 
was that the offspring of this cross (called the F1 generation, derived from the Latin term filius, meaning 
sons and daughters) were all tall plants. Based on the blending hypothesis, the result of all tall plants was 
unexpected. 

Next, Mendel let the F1 generation self-pollinate. He then noted that 75% of the resulting offspring, the F2 
generation, were tall, while 25% were short. Therefore, shortness appeared to have skipped a generation. 
Mendel found this same mathematical result over and over again with all the traits he studied. In all, Mendel 
studied seven characteristics, with almost 20,000 F2 plants analyzed. For example, purple flowers and white 
flowers were crossed to produce plants with only purple flowers in the F1 generation. Then after self-pollina- 
tion, the F2 generation had 75% purple flowers and 25% white flowers. These results did not reflect what 
you would expect if the blending model of inheritance was correct. 

Dominance 

Mendel had to come up with a new theory of inheritance to explain his results. His explanation, the law of 
segregation, is still one of the fundamental laws of modern genetics. He proposed that each pea plant had 
two hereditary factors for each trait. There were two possibilities for each hereditary factor, such as short 
or tall. One factor is dominant to the other, meaning it masks the effects of the recessive factor. However, 
each parent could only pass on one of these factors to the offspring. Therefore, during the formation of ga- 
metes, sperm or egg, the heredity factors must separate so there is only one factor per gamete. When fer- 
tilization occurs, the offspring then have two hereditary factors. 

This law explained what Mendel had seen in the F1 generation, because the two heredity factors were the 
short and tall factors and each individual in the F1 would have one of each factor, and as the tall factor is 
dominant to the short factor, all the plants appeared tall. In the F2 generation, produced by self-pollination 



112 



of the F1, 25% of the offspring could have two short heredity factors, so they would appear short. 75% would 
have at least one tall heredity factor and will be tall. 

In genetics problems the dominant factor is labeled with a capital letter (T) while the recessive factor is labeled 
with a lowercase letter (t). If we designate the letter Tor t to represent the heritable factor, as each individual 
has two factors for each trait, the possible combinations are 77, TT, and tt. Plants with TT would be tall while 
plants with tt would be short. Since 7" is dominant to t, plants that are Tt would be tall, as with the F1 gener- 
ation we described that inherited one factor from the TT tall parent and one factor from the tt short parent. 

Probability and Punnett Squares 

To visualize the results of a genetic cross, a Punnett square is helpful. Figure 3 is an example of a Punnett 
square that shows the results of a cross between two purple flowers that each have one dominant factor 
and one recessive factor (Bb). Notice how the possible factors in the sperm (6 or b) are lined up the side of 
the square while the possible factors in the egg (B or b) are lined up across the top. The possible offspring 
are represented by the letters in the boxes, with one factor coming from each parent. 





B 




B 




Bb 



pollen 

CT 





Figure 3: The Punnett Square. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Punnett_square_mendel_flowers.svg, License: GNU- 
FD) 

Notice how the Punnett square can help you predict the outcome of the crosses. Only one of the plants out 
of the four, or 25% of the plants, has white flowers (bb). The other 75% have purple flowers (BB, Bb) because 
the color purple is a dominant trait in pea plants. 

Now imagine you cross one of the white flowers (bb) with a purple flower that has both a dominant and re- 
cessive factor (Bb). The only possible gamete in the white flower is the recessive (b), while the purple flower 
can have gametes with either dominant (6) or recessive (b). If you write out the Punnett cross, you will see 
that 50% of the offspring will be purple and 50% of the offspring will be white. 





b 


b 


B 


Bb 


bb 


b 


Bb 


bb 



Figure 4: The Punnett Square defined. 



113 



(Source: Jessica Harwood, License: CC-BY-SA) 

Keep in mind that the birth of each offspring is an independent event and has the same probability, so the 
traits of a previous offspring do not influence the next offspring. In the cross discussed above with two Bb 
flowers, each offspring has a 75% chance of being purple and a 25% chance of being white. For example, 
even if the first three offspring in the cross have purple flowers, it does not mean that the next plant must 
have white flowers. All probability tells you is that overtime the averages of many, many offspring will work 
out to a predicted ratio. 

Lesson Summary 

• Gregor Mendel is considered the father of genetics, the science of studying inheritance. 

• According to Mendel's law of segregation, an organism has two factors for each trait, but each gamete 
only contains one of these factors. 

• A Punnett square is useful for predicting the outcomes of crosses. 

Review Questions 

1 . What is the term for the offspring of a cross, or the first generation? (Beginning) 

2. What is the F2 generation? (Intermediate) 

3. Who is considered the father of genetics? (Beginning) 

4. Why did Mendel select peas as a model for studying genetics? (Intermediate) 

5. In peas, yellow seeds (Y) are dominant over green seeds (y). If a yy plant is crossed with a YY plant, 
what ratio of plants in the offspring would you predict? (Challenging) 

6. In peas, purple flowers (P) are dominant over white flowers (p). If a pp plant is crossed with a Pp plant, 
what ratio of plants in the offspring would you predict? (Challenging) 

7. In guinea pigs, black fur (S) is dominant over white fur (b). If a BB guinea pig is crossed with a Bb"" guinea 
pig, what ratio of guinea pigs in the offspring would you predict? (Challenging) 

8. In guinea pigs, smooth coat (S) is dominant over rough coat (s). If a SS guinea pig is crossed with a ss 
guinea pig, what ratio of guinea pigs in the offspring would you predict? (Challenging) 

9. In humans, unattached ear lobes are dominant over attached ear lobes. If two parents have attached 
earlobes, what is the predicted ratio in the offspring? (Challenging) 

1Q Why would it be much easier to study genetics in pea plants than in people? (Intermediate) 

Further Reading I Supplemental Links 

http://www.mendelweb.org/MWtoc.html 

http://www.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgenintro.html 

http://www.mendelweb.org/MWtoc.html 

http://sonic.net/~nbs/projects/anthro201/ 

http://anthro.palomar.edu/mendel/mendel_1.htm 

Vocabulary 

dominant Masks the expression of the recessive trait. 



114 



F1 generation The first filial generation; offspring of the P or parental generation. 

F2 generation The second filial generation; offspring from the self-pollination of the F1 generation. 

gametes Haploid cells involved in sexual reproduction, such egg and sperm. 

genetics The study of inheritance. 

punnett square Visual representation of a genetic cross that helps predict the expected ratios in the off- 
spring. 

recessive Expression is masked by the dominant factor (allele); only expressed if both factors are 

recessive. 

Review Answers 

1. F1 generation 

2. The offspring from the F1 generation. In Mendel's studies, the F2 was created by self-pollinating F1 . 

3. Gregor Mendel 

4. Short generation time, easily identifiable traits. 

5. 100% Yy 

6. 50% white, 50% purple 

7. 50% Bb, 50% BB 

8. 100% smooth coat 

9. 100% attached earlobes 

10 Short generation time; you can't do controlled mating in people. 

Points to Consider 

• Do you think all traits follow this simple pattern where one factor controls the trait? 

• Can you think of other examples where Mendel's law does not seem to fit? 

Modern Genetics 

Lesson Objectives 

• Explain Mendel's laws with our modern understanding of chromosomes. 

• Explain how codominant traits are inherited. 

• Distinguish between phenotype and genotype. 

• Explain how polygenetic traits are inherited. 

Check Your Understanding 

• What is a visual representation of a genetic cross? 



115 



• What is stated in Mendel's law of segregation? 

Introduction 

Although Mendel laid the foundation for modern genetics, there were still a lot of questions left unanswered. 
How is inheritance determined for traits that do not seem to follow a simple dominant-recessive pattern? 
What exactly are the hereditary factors that determine traits in organisms? And how do these factors work? 
One of the great achievements of this past century was the discovery of DNA as the genetic material. And 
it is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge 
of DNA and chromosomes, we can explain Mendel's findings and build on them. 

Traits, Genes, and Alleles 

Interpreting Mendel's discoveries through the eye of modern genetics, we now know that Mendel's hereditary 
factors are made up of DNA. Recall that our DNA is wound into chromosomes. Each of our chromosomes 
contains a long chain of DNA that encodes hundreds, if not thousands, of genes. Each of these genes can 
have slightly different versions from individual to individual. These variants of genes are called alleles. For 
example, remember that for the height gene in pea plants there are two possible alleles, the dominant allele 
for tallness (T) and the recessive allele for shortness (t). 

Genotype and Phenotype 

Genotype refers to the combination of alleles that an individual has for a certain gene. For each gene, an 
organism has two alleles, one on each chromosome of a homologous pair of chromosomes. The genotype 
is often referred to with the letter combinations that were introduced in the previous lesson, such as TT, Tt, 
and tt. When an organism has two of the same alleles for a specific gene, it is homozygous for that gene. 
An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has 
two different alleles (Tt) for a certain gene, it is known as heterozygous. Genes have a specific place on a 
specific chromosome, so in the heterozygous individual these alleles are in the same location on each ho- 
mologous chromosome. 

Phenotype refers to the visible traits or appearance of the organism, as determined by the genotype. For 
example, the phenotypes of Mendel's pea plants were either tall or short, or were purple-flowered or white- 
flowered. Keep in mind that plants with different genotypes can have the same phenotype. For example, 
both a pea plant that is homozygous dominant for the tall trait (TT) and heterozygous plant (Tt) would have 
the phenotype of being tall plants. The recessive phenotype only occurs if the dominant allele is absent, 
which is when an individual is homozygous recessive (tt). 

Incomplete Dominance and Codominance 

In all of Mendel's experiments, he worked with traits where a single gene controlled the trait and where one 
allele was always dominant to the other. Although the rules that Mendel derived from his experiments explain 
many inheritance patterns, the rules do not explain them all. There are in fact exceptions to Mendel's rules, 
and these exceptions usually have something to do with the dominant allele. 

One exception to Mendel's rules is that one allele is always completely dominant over a recessive allele. 
Sometimes an individual has an intermediate phenotype between the two parents, as there is no dominant 
allele. This pattern of inheritance is called incomplete dominance. 

An example of incomplete dominance is the color of snapdragon flowers. One of the genes for flower color 
in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous 
for the red allele will have red flowers, while a plant that is homozygous for the white allele will have white 
flowers. On the other hand, the heterozygote will have pink flowers. Neither the red nor the white allele is 
dominant, so the phenotype of the offspring is a blend of the two parents. 



116 




Figure 1 : Pink snapdragons are an example of incomplete dominance. 

(Source: http://www.flickr.com/photos/tinfoilraccoon/1463287867/, License: CC-Attribution) 

Another example of incomplete dominance is sickle cell anemia, a disease in which the hemoglobin protein 
is produced incorrectly and the red blood cells have a sickle shape. A person that is homozygous recessive 
for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is ho- 
mozygous dominant will have normal red blood cells. And because this trait has an incomplete dominance 
pattern of expression, a person who is heterozygous for the sickle cell trait will have some misshapen cells 
and some normal cells. These heterozygous individuals have a fitness advantage; they are resistant to severe 
malaria. Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a 
combination of the recessive and dominant traits. 





Figure 2: Sickle cell anemia causes red blood cells to become misshapen and curved (upper figure) unlike 
normal, rounded red blood cells (lower figure). 



117 



(Sources: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Sicklecells.jpg, License: Public Domain; http://com- 
mons.wikimedia.org/wiki/lmage:Redbloodcells.jpg, License: Public Domain) 

An example of a codominant trait is ABO blood types, named for the carbohydrate attachment on the outside 

of the blood cell. In this case, two alleles are dominant and completely expressed (designated l A and l B ), 

while one allele is recessive (/'). The l A allele encodes for red blood cells with the A antigen, while the l B allele 
encodes for red blood cells with the B antigen. The recessive allele (/') doesn't encode for any antigens. An 
antigen is a substance that provokes an immune response, your body's defenses against disease, which 
will be discussed further in the Diseases and the Body's Defenses chapter. Therefore a person with two 

recessive alleles (if) has type O blood. As no dominant (l A and l B ) allele is present, the person cannot have 
type A or type B blood. 

There are two possible genotypes for type A blood, homozygous (l A l A ) and heterozygous (l A i), and two pos- 
sible genotypes for type B blood (l B i and l B l B ). If a person is heterozygous for both the l A and l B alleles, they 
will express both and have type AB blood with both antigens on each red blood cell. This pattern of inheritance 
is significantly different than Mendel's rules for inheritance because both alleles are expressed completely 
and one does not mask the other. 



Codominant 



Blood 
Type A 



Blood 
I TypeB 








Blood Type A BioodTypeAB Blood TypeB Blood Type 
(Codominant) 

U.S. National Library of Medicine 



Figure 3: An example of codominant inheritance is ABO blood types. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:C0d0minant.jpg, License: Public Domain) 

Polygenic Traits and Environmental Influences 

Another exception to Mendel's rules is polygenic inheritance, which is when a trait is controlled by more 
than one gene. Often these traits are in fact controlled by many genes on many chromosomes. Each dominant 
allele has an additive effect, so the resulting offspring can have a variety of genotypes, from no dominant 
alleles to several dominant alleles. In humans, some examples of polygenic traits are height and skin color. 
People are neither short nor tall, as was seen with the pea plants studied by Mendel, which has only one 
gene that encodes for height. Instead, people have a range of heights determined by many genes. Similarly, 
people have a wide range of skin colors. Polygenic inheritance often results in a bell shaped curve when 
you analyze the population (Figure 4). That means that most people are intermediate in the phenotype, 



118 



such as average height, while very few people are at the extremes, such as very tall or very short. 




Figure 4: Polygenic traits tend to result in a distribution that resembles a bell-shaped curve, with few at the 
extremes and most in the middle. There may be 4 or 6 or more alleles involved in the phenotype. At the left 
extreme, individuals are completely dominant for all alleles, and at the other extreme, individuals are com- 
pletely recessive for all alleles. Individuals in the middle have various combinations of recessive and dominant 
alleles. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Bellcurve.svg, License: Public Domain) 

Most polygenetic traits are partially influenced by the environment. For example, height is partially influenced 
by nutrition in childhood. If a child is genetically programmed to be average height but does not get a proper 
diet, he or she may be below average in size. 

Other examples of environmentally influenced traits are mental illnesses like schizophrenia and depression. 
A person may be genetically predisposed to have depression, so when that person's environment contributes 
major stresses like losing a job or losing a close relative, the person is more likely to become depressed. 

Lesson Summary 

• Variants of genes are called alleles. 

• Genotype is the combination of alleles that an individual has for a certain gene, while phenotype is the 
appearance caused by the expression of the genotype. 

• Incomplete dominance and codominance do not fit Mendel's rules because one allele does not entirely 
mask the other. 

• In polygenic inheritance, many genes control a trait with each dominant allele having an additive effect. 

Review Questions 

1 . What is a variant of a gene that occurs at the same place on homologous chromosomes? (Beginning) 

2. What is the type of allele that only affects the phenotype in the homozygote condition? (Beginning) 

3. What type of allele masks the expression of the recessive allele and is therefore expressed in the het- 
erozygote? (Beginning) 

4. What is the term for the specific alleles of an individual for a particular trait? (Beginning) 

5. What is the term for the appearance of the organism, as determined by the genotype? (Beginning) 

6. If a organism has a certain phenotype, such as a tall pea plant, does that mean it must have the same 
genotype? (Intermediate) 



119 



7. What is the term for the pattern of inheritance where an individual has an intermediate phenotype between 
the two parents? (Beginning) 

8. IQ in humans varies in humans with most people having an IQ of around 100, and with a few people at 
the extremes, such as 50 or 1 50. What type of inheritance do you think this might describe? (Challenging) 

9. A dark purple flower is crossed with a white flower of the same species and the offspring have light purple 
flowers. What type of inheritance does this describe? (Intermediate) 

10 What is the inheritance pattern where both alleles are expressed? (Beginning) 

Further Reading I Supplemental Links 

• http://en.wikipedia.org/wiki/Dominant_gene 

• http://en.wikipedia.org/wiki/Polygenic_inheritance 

• http://staff.jccc.net/pdecell/evolution/polygen.html 

• http://www.curiosityrats.com/genetics.html 

• http://www.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgenintro.html 

• http://www.curiosityrats.com/genetics.html 

• http://www.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgenintro.html 

Vocabulary 

allele An alternative form of a gene. 

co-dominance A pattern of inheritance where both alleles are equally expressed. 

genotype The genetic makeup of a cell or organism, defined by certain alleles for a partic- 

ular trait. 

heterozygous Having identical alleles for a particular trait. 

homozygous Having two different alleles for a particular trait. 

incomplete dominance A pattern of inheritance where the offspring has a phenotype that is halfway be- 
tween the two parents' phenotypes. 

phenotype The physical appearance that is a result of the genotype. 

polygenic inheritance A pattern of inheritance where the trait is controlled by many genes and each 

dominant allele has an additive effect. 

Review Answers 

1. allele 

2. recessive allele 

3. dominant allele 

4. genotype 

5. phenotype 

6. No, the homozygous dominant and heterozygous would have the same phenotype. 

7. incomplete dominance 

8. polygenic inheritance, since it follows a bell curve 



120 



9. incomplete dominance 
1Q codominance 

Points to Consider 

• Hypothesize about the genetic differences between males and females. 

• Can you name any human genetic disorders? 

• If a baby inherits an extra chromosome, what might the result be? 

Human Genetics 

Lesson Objectives 

• List the two types of chromosomes in the human genome. 

• Predict patterns of inheritance for traits located on the sex chromosomes. 

• Describe how some common human genetic disorders are inherited. 

• Explain how changes in chromosomes can cause disorders in humans. 

Check Your Understanding 

• How many alleles does an individual have for each gene/trait? 

• How do we predict the probability of traits being passed on to the next generation? 

• What do we call complexes of DNA wound around proteins that pass on genetic information to the next 
generation of cells? 

Introduction 

You might know someone who was born with a genetic disorder, such as cystic fibrosis or Down syndrome. 
And you might have wondered how someone inherits these types of disorders. It all goes back to Mendel! 
Mendel's rules laid the foundation for understanding the genetics of all organisms, including humans. We 
can apply Mendel's rules to describe how many human traits and genetic disorders are inherited. Some 
disorders are caused by a single recessive allele, while other disorders are caused by a single dominant 
allele. Therefore, we can draw a Punnett square to predict the number of offspring that may be affected with 
these diseases, just like we predicted for other traits in the previous lessons. Since Mendel's time, we have 
also expanded our knowledge of inheritance and understand that genes are located on chromosomes. Now 
we can now explain special inheritance patterns that don't fit Mendel's rules. 

Sex-linked Inheritance 

What determines if a baby is a boy or a girl? Recall that you have 23 pairs of chromosomes, one pair of 
which are the sex chromosomes. Everyone has two sex chromosomes, X or Y, that determine our sex. Fe- 
males have two X chromosomes, while males have one Y chromosome and one X chromosome. So if a 
baby inherits an X from the father and an X from the mother, it will be a girl. If the father's sperm carries the 
Y chromosome, it will be a boy. Notice that a mother can only pass on an X chromosome, so the sex of the 
baby is determined by the father. The father has a 50 percent chance of passing on the Y or X chromosome, 
hence it is a 50 percent chance whether a child will be a boy or a girl. 

One special pattern of inheritance that doesn't fit Mendel's rules is sex-linked inheritance, referring to the 
inheritance of traits which are due to genes located on the sex chromosomes. The X chromosome and Y 
chromosome carry many genes and some of them code for traits that have nothing to do with determining 



121 



sex. Since males and females do not have the same sex chromosomes, there will be differences between 
the sexes in how these sex-linked traits are expressed. 

One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot 
distinguish between red and green and often see these colors as shades of brown. Boys are much more 
likely to be colorblind than girls. That's because colorblindness is a sex-linked recessive trait. Boys only 
have one X chromosome, so if that chromosome carries the gene for colorblindness, they will be colorblind. 
As girls have two X chromosomes, a girl can have one X chromosome with the colorblind gene and one X 
chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal 
gene will mask the recessive colorblind gene. For a girl to be colorblind, she would have to inherit two genes 
for colorblindness, which is very unlikely. Many sex-linked traits are inherited in a recessive manner. 




Figure 1: A person with red-green colorblindness would not be able to see the number. 

(Source: http://en.wikipedia.Org/wiki/lmage:lshihara_9.png, License: Public Domain) 

A woman can be a carrier of colorblindness, however. A carrier appears normal but is capable of passing 
on a genetic disorder to her child. Carriers for colorblindness have a heterozygous genotype of one colorblind 
allele and one normal allele. We can use a Punnett square to predict the probability of a carrier passing on 
the trait to her children. For example, if a woman who is a carrier for colorblindness has children, her boys 
would have a 50% chance of being colorblind and her girls have a 50% chance of being carriers. 





Xc 


X 


X 


XcX 

(carrier female) 


XX 

(normal female) 


Y 


XcY 

(colorblind male) 


XY 

(normal male) 



122 



Figure 2: According to this Punnett square, the son of a woman who carries the colorblindness trait and a 
normal male has a 50% chance of being colorblind. 

Human Genetic Disorders 

Some human genetic disorders are also X-linked or Y-linked, which means the faulty gene is carried on 
these sex chromosomes. Other genetic disorders are carried on one of the other 22 pairs of chromosomes; 
these chromosomes are known as autosomes or autosomal chromosomes. 

Some genetic disorders are caused by recessive or dominant alleles of a single gene on an autosome. 
These disorders would then have the same inheritance pattern as any other dominant or recessive trait. An 
example of an autosomal recessive genetic disorder is cystic fibrosis. Children with cystic fibrosis have ex- 
cessively thick mucus in their lungs which makes it difficult for them to breathe. The inheritance of this re- 
cessive allele is the same as any other recessive allele, so a Punnett square can be used to predict the 
probability that two carriers of the disease will have a child with cystic fibrosis. 





F 


f 


F 


FF 

(normal) 


Ff 

(carrier) 


f 


Ff 

(carrier) 


ff 

(affected) 



Figure 3: According to this Punnett square, two parents that are carriers of cystic fibrosis of the cystic fibrosis 
gene have a 25% chance of having a child with cystic fibrosis. 

Another recessive trait that we mentioned previously was sickle cell anemia. A person with two recessive 
alleles for the sickle cell trait (aa) will have sickle cell disease. In this disease the hemoglobin protein is 
formed incorrectly and the person's red blood cells are misshapen. A person who does not carry the sickle 
trait has a homozygous dominant genotype (AA). Remember the trait showed incomplete dominance, so a 
person who is heterozygous for the trait (Aa) would have some sickle-shaped cells and some normal red 
blood cells. 

You can also use a simple Punnett square to predict the inheritance of a dominant autosomal disorder, like 
Huntington's disease. If one parent has Huntington's disease, what is the chance of passing it on to their 
children? If you draw the Punnett square, you will see that there is a 50 percent chance of the disorder being 
passed on to the children. Huntington's disease causes the brain's cells to break down, leading to muscle 
spasms and personality changes. Unlike most other genetic disorders, the symptoms usually do not become 
apparent until middle age. 

Genetic diseases can also be carried on the sex-chromosomes. An example of a recessive sex-linked genetic 
disorder is hemophilia. A hemophiliac's blood does not clot, or clots very slowly, so he or she can easily 
bleed to death. As with colorblindness, males are much more likely to be hemophiliacs since the gene is on 
the X chromosome. Because Queen Victoria of England was a carrier of hemophilia, this disorder was once 
common in European royal families. Several of her grandsons were afflicted with hemophilia, but none of 
her granddaughters were affected by the disease, although they were often carriers. Because at the time 
medical care was very primitive, often hemophiliacs bled to death, and usually at a young age. Queen Vic- 
toria's grandson Frederick died at age 3, and her grandson Waldemar died at age 1 1 . 



123 



The British Haemophilia Line 



Kev 

X: Unaffected X- chromosome 

Y: Y-chifQinQ'SGiTie 

x. Affected X-chromDsomB 



Oueen Victoria 
Xx 



T 



Albert of Sax e-Coburg-Golria 
XY 



Wctoria 
XX 



Vicloria 
XX 



King Edward VII 
XY 



Alice 
X* 



Grand Duke Louis IV 

of Hessa 
EL 



1 1 - 

Alfred Helena 
XY XX? 



Louise 
XX? 



Arthur 
XY 



_L 



s.V 



Leopold i-Helene of 



Elisabeth 
XX? 



X* 



Waldemar 
«Y 



Irene -r— Hertry of 



Prussia 
XY 



Grand Duke 

Erne si Louis 

XY 



~r 



Frederick 
iY 



impetor Nicholas II 



AIih | — i — of Russia 



J_ 



Al c e 
Xx 



'-'■ >;i' ■■! i 1 1 
XY 



remv 
xY 



_ I :| a 
!ffl? 



Tatiana 
XX? 



Mane Anaslasia 
XX? XX? 



Alexer 
xY 



Waldeck 
XX 

Alexander 

Cambridge, 

1 st Ear) 

of Afhlorte 

XY 



Henry of 
Bealrice k- Bsltenbarg 
Xx 






Rupel 
xY 



Maurice 
xY 



I 

Alexander Mountbatlen, 
1st Marquess of Carisbrooke 
XY 



T 



X« 



Victoria Eugenie -p King Alfonso XIII 



Alfonso Jaime 
sY XY 



1 — 

Bealri; 
XX 



of Spain 

XY 



Leopold 
iY 



Maurice 

xY? 



Crietina Gonzalo 
XX xY 



Figure 4: A pedigree chart shows all the phenotypes for a particular trait in the family. This pedigree chart 
traces back the occurrence of hemophilia in the British royal family. Those individuals with boxes around 
them are either female carriers of the trait or males inflicted with the trait. 

(Source: http://en.wikipedia.Org/wiki/lmage:Haemophilia_family_tree.gif, License: Public Domain) 

Many genetic disorders are recessive, meaning that an individual must be homozygous for the recessive 
allele to be affected. Sometimes these disorders are lethal (deadly), however, heterozygous individuals 
(unaffected individuals with one dominant allele and one recessive allele) survive. This allows the allele that 
causes the genetic disorder to be maintained in a population's gene pool. A gene pool is the complete set 
of unique alleles in a species or population. A mutation is a change in the DNA sequence. New mutations 
are constantly being generated in a gene pool. 

Chromosomal Disorders 

Some children are born with genetic defects that are not carried by a single gene. Instead, an error in a 
larger part of the chromosome or even in an entire chromosome causes the disorder. Usually the error 
happens when the egg or sperm is forming. One common example is Down syndrome. Children with Down 
syndrome are mentally disabled and have collection of recognizable physical deformities. Down syndrome 
occurs when a baby receives an extra chromosome from one of his or her parents. Usually a child would 
have one chromosome 21 from your mother and one chromosome 21 from your father. But in an individual 
with Down syndrome, there are three copies of chromosome 21 . Down syndrome is also known as trisomy 
21. 



124 




Figure 5: A child with Down syndrome. 

(Source: http://flickr.com/photos/lobberich/2009136373/, License: CC-BY) 

Another example of a chromosomal disorder is Klinefelter syndrome, in which a male inherits an extra "X" 
chromosome. These individuals have underdeveloped sex organs and elongated limbs, and have difficulty 
learning new things. Outside of chromosome 21 and the sex chromosomes, most embryos with multiple 
chromosomes do not even make it to the fetal stage. Because chromosomes carry many, many genes, a 
disruption of a chromosome potentially causes severe problems with development of the fetus. 

Besides diseases caused by duplicated chromosomes, other chromosomal disorders occur when the 
structure of a chromosome is disrupted. For example, if a tiny portion of chromosome 5 is missing, the indi- 
vidual will have cri du chat (cat's cry) syndrome. These individuals have misshapen facial features and the 
infant's cry resembles a cat's cry. 

Lesson Summary 

• Some human traits are controlled by genes on the sex chromosomes. 

• Human genetic disorders can be inherited through recessive or dominant alleles, and they can be located 
on the sex chromosomes or autosomes. 

• Changes in chromosome number can lead to disorders like Down syndrome. 

Review Questions 

1 . How many chromosomes do you have in each cell of your body? (Beginning) 

2. How is Down syndrome inherited? (Intermediate) 

3. A son cannot inherit colorblindness from his father. Why not? (Challenging) 



125 



4. One parent is a carrier of the cystic fibrosis gene, while the other parent does not carry the allele. Can 
their child have cystic fibrosis? (Challenging) 

Further Reading I Supplemental Links 

• http://www.articlesbase.com/health-articles/what-is-haemophilia-412305.html 

• http://www.scribd.eom/doc/1 01 8249/lectureChromsomes-and-Human-Genetics-Guevedoces 

• http://geneticdisorderinfo.wikispaces.com/ 

• http://learn.genetics.utah.edu/units/disorders/karyotype/karyotype.cfm 

• http://www.hhmi.org/biointeractive/vlabs/cardiology/index.html 

• http://en.wikipedia.org/ 

Vocabulary 

autosomes The chromosome other than the sex chromosomes. 

carrier A person who is heterozygous for a recessive genetic disorder; the person does not have 

the disease but can pass the disease allele to the next generation. 

sex-linked trait A trait that is due to a gene located on a sex chromosome, usually the X-chromosome. 

Review Answers 

1. 23 pairs 

2. Through an error in egg or sperm formation, resulting in an individual with an extra chromosome 21 . 

3. Colorblindness is carried on the X chromosome. A boy gets an X chromosome from his mother and a Y 
chromosome from his father. If his father passed on the faulty X chromosome, the child would be a girl! 

4. No. This is a recessive disorder. 

Points to Consider 

• Human cloning is illegal in many countries. Do you agree with these restrictions? 

• Why would it be helpful to know all the genes that make up human DNA? 

• It may be possible in the future to obtain the sequence of all your genes. Would you want to take advantage 
of this opportunity? Why or why not? 

Genetic Advances 

Lesson Objectives 

• Explain how clones are made. 

• Explain how vectors are made. 

• Explain what sequencing a genome tells us. 



126 



• Describe how gene therapy works. 

Check Your Understanding 

• What part of the cell contains the genetic material? 

• What are the base pairing rules for DNA? 

Introduction 

Since Mendel's time, there have been rapid advances in the understanding of genetics. As scientists under- 
stand better how DNA works, they can develop technologies that allow us to reveal the genetic secrets en- 
coded in our DNA and even alter an organism's DNA. Genetic engineering (or biotechnology or DNA tech- 
nology) has helped us better understand and predict the inheritance of genetic diseases, produce new 
medicines, and even produce new food products. DNA technology has also made an impact on fighting 
crime. Because DNA is unique to an individual, the DNA in just a few hairs at a crime scene can help identify 
a criminal. This technology, known as DNA fingerprinting, has also helped innocent imprisoned people to 
appeal their case and clear their names. DNA technology has revolutionized not only criminal justice, but 
also many other aspects of our lives. 

Recombinant DNA 

Recombinant DNA is the combination of DNA from two different sources. It is useful in gene cloning and 
in identifying the function of a gene, as well as producing useful proteins. Human insulin for treating diabetes 
has been produced through recombinant DNA methods. In this process, a gene of interest (or piece of DNA 
of interest) is placed into a host cell, such as a bacterium, so the gene can be copied (and cloned) and the 
protein that results from that gene can be produced. 

To place the gene of interest into a host cell, a vector, or carrier molecule, is needed to the carry foreign 
DNA into the host cell. Bacteria have small accessory rings of DNA in the cytoplasm, called plasmids. When 
putting foreign DNA into a bacterium (a host cell), the plasmids are often used as a vector. Viruses can also 
be used as vectors. 

The first step of making recombinant DNA involves a restriction enzyme that cuts the vector and the foreign 
(exogenous) DNA. Restriction enzymes cut DNA at specific sequences, such as GAATTC as shown in 
Figure 1 . There are more than 3000 known restriction enzymes, most cutting the DNA at a unique sequence. 
This reaction results in the plasmid opening up a gap with "sticky ends", which can attach with the compli- 
mentary base pairs on the sticky ends of the foreign DNA. Then the enzyme DNA ligase seals the foreign 
DNA in its new place inside the plasmid. These altered plasmids are introduced back into the bacteria, a 
process called transformation. The bacteria will express the foreign gene. 



GAATTC 
CTTAAG 



Figure 1: Restriction enzymes cut DNA at specific sequences, in this example the sequence "GAATTC". 
The enzyme cuts between the G and A on each strand, producing overhanging "sticky ends." 

(Source: http://commons.wikimedia.Org/wiki/lmage:Restriction_enzyme.jpg, License: Public Domain) 



127 




Figure 2: A plasmid can be used to introduce a foreign (or exogenous) gene (blue) into bacteria. The other 
segment of foreign DNA (green) encodes for antibiotic resistance, which allows for selection of the transformed 
bacteria by growing them on plates containing antibiotic. Only the bacteria that have not picked up the 
plasmid will die from the antibiotic. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Example_plasmid.png, License: GNU-FD) 

One application of recombinant DNA technology is producing the protein insulin, which is needed to treat 
diabetes. Previously, insulin had been extracted from the pancreases of animals. Through recombinant DNA 
technology, bacteria were created that carry the human gene which codes for the production of insulin. 
These bacteria become tiny factories that produce this protein. 

Cloning 

Cloning is the process of creating an exact replica of an organism. The clone's DNA is exactly the same as 
the parent's DNA. Bacteria and plants have long been able to clone themselves through processes of 
asexual reproduction. In animals, however, cloning does not happen naturally. 

Animals can now be cloned in a laboratory, however. In 1997, a sheep named Dolly was the first mammal 
ever to be successfully cloned. The process of producing an animal like Dolly starts with a single cell from 
the animal that is going to be cloned. In the case of Dolly, cells from the mammary glands were taken from 
the adult that was to be cloned. These cells are called somatic, meaning they come from the body and are 
not gametes like sperm or egg. Remember that somatic cells have a diploid number of chromosomes. Next, 
the nucleus was removed from this cell. The nucleus was placed in a donor egg that had already had the 
nucleus removed. The new cell then divided after the stimulation of an electric shock, and development 
proceeded normally just as if the embryo had formed naturally. The resulting embryo was implanted in a 
surrogate mother sheep, where it continued its development. 



Somatic body cell with desired genes 

Nucleus fused with denucleated egg cell 



-> • 



t 

o 



Clone 



Egg cell 

Nucleus removed 

reproductive cloning (SS» — ► Surrogate mother 



Figure 3: To clone an animal, a nucleus from the animal's cells are fused with an egg cell (in which the 
nucleus has been removed) from a donor. 



128 



(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:CI0ning_diagram_english.png, License: GNU-FD) 

Cloning is not always successful, though. Most of the time, this cloning process does not result in a healthy 
adult animal. The process has to be repeated many times until it works. In fact, 277 tries were needed to 
produce Dolly. This high failure rate is one reason that human cloning is banned in the United States. In 
order to produce a cloned human, many attempts would result in the surrogate mothers experiencing mis- 
carriages, stillbirths, or deformities in the infant. There are also many additional ethical considerations related 
to human cloning. 

Human Genome Project 

A person's genome is all of his or her genetic information; in other words, the human genome is all the infor- 
mation that makes us human. The Human Genome Project was an international effort to sequence all 3 
billion base pairs that make up our DNA and to identify within this code the over 20,000 human genes. Sci- 
entists also completed a chromosome map, identifying where the genes are located on each of the chromo- 
somes. The Human Genome Project was completed in 2003. Though the Human Genome Project is finished, 
analysis of the data will continue for many years. 



12 3 4 5 

i( K (c ^ fl ?} v 



10 11 12 



A /<- II M V it 

13 14 15 16 17 IB 

II K ,1 ri J 1 

19 20 21 22 X/V 



Figure 4: To complete the Human Genome Project, all 23 pairs of chromosomes in the human body were 
sequenced. Each chromosomes contains thousands of genes. This is a karyotype, a visual representation 
of an individual's chromosomes lined up by size. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Human_male_karyotpe_high_resolution.jpg, License: 
Public Domain) 

There are many exciting applications of the Human Genome Project. The genetic basis for many diseases 
can be more easily determined, and now there are tests for over 1 ,000 genetic disorders. The National In- 
stitutes of Health, the United States government's premiere biomedical research community, is also looking 
for ways to reduce the costs of sequencing so that people can have a map of their individual genome. Although 
some disorders are caused by a single gene, many other illnesses are caused by a combination of several 
genes and a person's lifestyle. Analysis of your own genome could determine if you are at risk for specific 
diseases. Knowing you might be genetically prone to a certain disease would allow you to better seek pre- 
ventive lifestyle changes and medical screenings. 

A genetic map shows the location (or loci) of a gene on a chromosome. Genetic maps are important tools 
to help researchers understand genes and genetic diseases. Knowing where genes are in relation to other 
genes and knowing the order of genes on a chromosome is an important aspect of human genetics. The 



129 



frequency of recombination (crossing-over during prophase I of meiosis) allows geneticists to estimate the 
distance between loci. Because crossing-over occurs relatively rarely at any location along the chromosome, 
the frequency of recombination between two locations depends on their distance. The farther apart genes 
are on the same chromosome, the more likely there is to be a cross-over event between them. The likelihood 
of a cross-over event between two closely located genes (said to be linked) is small. 

Gene Therapy 

Gene therapy is the insertion of genes into a person's cells to cure a genetic disorder. There are two main 
types of gene therapy; one done inside the body and one done outside the body. In ex vivo gene therapy, 
done outside the body, cells are removed from the patient and the proper gene is inserted using a virus as 
a vector. Then the modified cells are placed back into the patient. One of the first uses of this type of gene 
therapy was in the treatment of a young girl with a rare genetic disease, Adenosine deaminase deficiency, 
or ADA deficiency. People with this disorder are missing the ADA enzyme, which breaks down a toxin called 
deoxyadenosine. If the toxin is not broken down, it accumulates and destroys immune cells. As a result, in- 
dividuals with ADA deficiency do not have a healthy immune system to fight off infections. In the gene 
therapy treatment for this disorder, bone marrow stem cells were taken from the girl's body and the missing 
gene was inserted in these cells outside the body. Then the modified cells were put back into her bloodstream. 
This treatment proved sufficient to restore the function of her immune system, but only with continual repeated 
treatments. 

During in vivo gene therapy, done inside the body, the vector with the gene of interest is introduced directly 
into the patient and taken up by the patient's cells. The vector is inserted where the gene product is needed. 
For example, cystic fibrosis gene therapy is targeted at the respiratory system, so a solution with the vector 
can be sprayed into the patient's nose. Recently in vivo gene therapy was also used to partially restore the 
vision of three young adults with a rare type of retinal disease that is congenital, meaning present at birth. 

Lesson Summary 

• Using recombinant DNA technology, a foreign gene can be inserted into an organism's DNA. 

• Cloning of mammals is still being perfected, but several cloned animals have been created by implanting 
the nucleus of a somatic cell into a cell in which the nucleus has been removed. 

• The Human Genome Project produced a genetic map of all the human chromosomes and determined 
the sequence of every base pair in our DNA. 

• Gene therapy involves treating an illness caused by a defective gene through the use of a vector to inte- 
grate a normal copy of the gene into the patient. 

Review Questions 

1 . What is the enzyme used to cut DNA at specific points? (Intermediate) 

2. What is the term for all the genetic information of the human species? (Intermediate) 

3. What are the rings of accessory DNA in bacteria that are often used as a vector in genetic engineering? 
(Beginning) 

4. What is the term for producing identical copies of an organism? (Beginning) 

5. Can gene therapy cure a disease caused by a virus? (Challenging) 

6. What is the vehicle used to introduce foreign DNA into an organism? (Beginning) 

7. What is one disease that genetic therapy can help treat? (Intermediate) 

8. What supplies the cytoplasm of the clone's cells during the cloning of an organism? (Challenging) 

9. What is one application of recombinant DNA technology? (Intermediate) 



130 



1Q Is gene therapy for ADA deficiency a permanent fix? (Intermediate) 

Further Reading I Supplemental Links 

• http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml 

• http://history.nih.gov/exhibits/genetics/sect4.htm 

• http://learn.genetics.utah.edu/units/disorders/whataregd/ada/ 

• http://www.lifesitenews.com/ldn/2007/nov/07112003.html 

• http://www.le.ac.uk/ge/genie/vgec/sc/genomics.html 

• http://en.wikipedia.org/wiki/Recombinant_DNA 

• http://www.hhmi.org/biointeractive/vlabs/transgenic_fly/index.html 

• http://www.groundreport.com/World/Scientists-to-clone-rhino 

Vocabulary 



cloning Creating an identical copy of an individual with the same genes. 

DNA ligase Enzyme that joins DNA fragments together. 

gene therapy Treatment that provides a new gene to replace a defective gene; potentially 

"cures" a genetic disease. 

human genome project International effort to sequence all the base pairs in human DNA. 



plasmid 
recombinant DNA 

somatic cell 
transformation 

vector 



An accessory circle of DNA in bacteria. 

DNA formed bu the combination of DNA from two different sources, such as 
placing a human gene into a bacterial plasmid. 

A body cell; not a gamete. 

The process by which bacteria pick up foreign DNA and incorporate it in their 
genome. 

A vehicle, such as a plasmid, used to transfer foreign DNA into an organism. 



Review Answers 

1 . restriction enzyme 

2. the human genome 

3. plasmids 

4. cloning 

5. No, gene therapy can only cure a genetic disorder. 

6. a vector (such as a plasmid) 

7. cystic fibrosis, ADA deficiency, etc. 

8. donor egg 

9. insulin production 



131 



1Q No, treatments have to be repeated throughout the person's lifetime. 
Points to Consider 

Next we begin to discuss evolution, the change in species over time. 

• Fossils provide evidence of evolution, but what is a fossil? 

• If two animals are similar in structure, would you guess they are closely related? Why or why not? 



132 



7. Evolution 



Evolution by Natural Selection 

Lesson Objectives 

• Understand that inherited traits, such as the basic color of skin or a person's bone structure, are passed 
on to future generations. 

• Understand that acquired traits, such as a tan or being good at soccer, are not passed on to future 
generations (they are not inherited). 

• Understand that evolution is change of an inherited trait in a population over many generations, such as 
the change of the color of moths living on an island over many generations. 

• Understand that natural selection means that organisms with traits that help them survive in their envi- 
ronment are more likely to survive than organisms without that beneficial trait. 

• Understand how evolution explains: 



Why populations change. 



Why there are so many different kinds of organisms on earth. 



Why some organisms that look alike only distantly related. 



• Why some organisms that look very different actually closely related? 

• Know that both Darwin and Wallace developed the theory of evolution by natural selection at the same 
time. 

Check Your Understanding 

• What does the word evolution refer to when used in day to day conversations? 

• What does biological evolution mean? 

• Who primarily proposed the theory of evolution by natural selection? 

Introduction 

Biological evolution is change in species over time. The idea of evolution was proposed by many other 
people before Darwin began collecting evidence for the idea. Scientists for hundreds of years had hypothe- 
sized that species change over time. But it was not until Darwin published his research and detailed analysis 
that the idea of evolution started to gain widespread acceptance. Darwin's theory of evolution by natural 
selection brings all fields of biology together and illuminates nearly every aspect of biology. As one famous 
biologist said, "Nothing in biology makes sense except in the light of evolution." 



133 




Figure 1: Charles Darwin was one of the most influential scientists who has ever lived. Darwin introduced 
the world to the theory of evolution by natural selection which laid the foundation for how we understand the 
living world today. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Charles_Darwin_by_G._Richmond.jpg, License: Public Domain) 

Evolution by natural selection explains: 



• The tremendous variety of organisms on Earth. 

• Why some organisms that resemble each other are distantly related. 

• Why some organisms that do not resemble each other are closely related. 
There are three parts to Darwin's Theory of Evolution by Natural Selection. 

1 . Evolution, which is change in species over multiple generations. 

2. Natural selection, in which individuals of a population that are most likely to survive and reproduce are 
also most likely to pass on traits that have a genetic basis to any offspring. 

3. Adaptation, which are traits that help a plant or animal survive and reproduce in a particular environment. 
Adaptations are the result of natural selection. For example, light-colored moths on dark trees might be 
easier for birds to see and catch than dark moths on dark-colored trees. If the moths' color has a genetic 
basis, then after many generations of birds catching more light moths than dark moths, the population of 
moths will consist mostly of dark moths. 



134 




Gibbon. 



Orano. 



Skeleton* of the 
Chimpanzee. 



Gorilla. 



Man 



Photographically reduced from Diagrams of the natural size (excejtt that of the Gibtnm, which teas twice an large as naturrj, 
drawn by Mr. Waterhouse Hawkins from sj>ecimens in the Museum of the Uoyal College of Surgeons. 



Figure 2: Humans and the other apes in this drawing all evolved from a common apelike ancestor. 

In everyday English, "evolution" simply means to "change" or "stepwise change from simple to complex." In 
biology, evolution means change in the inherited traits of a group of organisms over multiple generations. 
Biological evolution has changed biologists' understanding of all life on Earth. 





Figure 3: Human earlobes may be free or attached. You inherited the particular shape of your earlobes 
from your parents. Inherited traits are influenced by genes, which are passed on to offspring and future 
generations. Your summer tan is not passed on to your offspring. Natural selection only operates on traits 
like earlobe shape that have a genetic basis, not on traits like a summer tan that are "acquired." 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/6/68/Earlobes_free_attached.jpg) 

Darwin's Observations 

Most people in the world did not become aware of the theory of evolution until 1 859, when Charles Darwin 
published his book On the Origin of Species Means of Natural Selection. This book described the observations 
and evidence that he collected over 20 years of intensive research, beginning with a five-year voyage around 
the world on a British research ship, the HMS Beagle. Darwin embarked on this five-year voyage when he 
was during this trip, on which Darwin used this trip to make observations about plants and animals spread 
around the world and to collect specimens to study when he returned to England. Each time the Beagle, 
stopped at a port to do some trading, Darwin went on land to explore and look for the local plants, animals, 
and fossils. One of the most important things Darwin did was to keep a diary. He made extremely detailed 
notes and drawings about everything he saw as well as his thoughts. 



135 




Figure 4: Charles Darwin's famous five year voyage was aboard the HMS Beagle from 1831-1836. 

(Source: http://en.wikipedia.0rg/wiki/lmage:V0yage_0f_the_Beagle.jpg, License: GNU Free Documentation) 

The around the world voyage of the HMS Beagle was mostly to map the coastline of South America. Darwin's 
best known discoveries were made on the Galapagos Islands, a group of 16 volcanic islands near the 
equator about 600 miles from the west coast of South America. Darwin was able to spend months on foot 
exploring the islands. Darwin's Theory of Evolution by Natural Selection was a result of his observations 
and 20 years of examining the specimens he had collected and sent back to England, many of which came 
from these islands. 




Figure 5: The Galapagos Islands are a group of 16 volcanic islands 972 km off the west coast of South 
America. The islands are famous for their many species found nowhere else. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Galapagos-satellite-esislandnames.jpg, License: Public Domain) 

Darwin was amazed by the array of life he saw on these barren islands. He saw animals unlike anything he 
had ever seen before. Darwin was struck by how the same kind of animal differed from one island to another. 



136 



For example, the iguanas (large lizards) differed between islands. The members of one species spent much 
of theirtime swimming and diving underwater for seaweed, while those of another species lived on land and 
ate cactus. 




Figure 6: The Galapagos land iguanas are among the signature animals of the Galapagos Islands. 

(Source: http://en.wikipedia.Org/wiki/lmage:Galapagos_iguana.jpg, License: Public Domain) 

In England, he was accustomed to watching cormorants fly, so he was surprised to find flightless cormorants 
on the islands alongside flying cormorants. Giant tortoises, large enough for two men to ride on, plodded 
across the islands and foraged on super tough leaves. Some of the tortoise species were found on only one 
island. Darwin was fascinated by the number of ways that organisms were well-suited to their environments. 
Even the tortoise shells were specially adapted to the conditions. Tortoises that ate plants near the ground 
had rounded shells, while the tortoises that stretched their necks to reach plants higher in shrubs had shells 
that bent upwards, allowing them to stretch their necks upward. 




Figure 7: The name "Galapagos" means "giant tortoise." When Darwin arrived on the Galapagos Islands, 
he was amazed by the size and variety of shapes of these animals. The tortoise is a unique animal found 
only in the Galapagos Islands. There only about 200 tortoises remaining on these islands. 

(Source: http://en.wikipedia.Org/wiki/lmage:Gal%C3%A1pagos_Giant_Tortoise.jpg) 



137 




Figure 8: This tortoise is able to reach leaves high in shrubs with its long neck and curved shell. 

(Source: http://texasturtle.files.wordpress.com/2008/08/lonesome-george.jpg) 

The most extensively studied animals on the Galapagos are the finch species (birds). When Darwin first 
observed the finches on the islands, he did not even realize they were all finches. But when he studied them 
further, he realized they were all the same type of bird, and that each island had its own distinct species of 
finch. The birds on different islands had many similarities, but their beaks differed in size and shape. 




1. Geospiza rnagnirostris 
3. Geospiza parvula 



2. Geospiza Fortis 
4. Certhidea olivacea 



Finches from Galapagos Archipelago 



Figure 9: Four of Darwin's finch species from the Galapagos Islands. The birds came from the same finch 
ancestor. They evolved as they adapted to different food resources on different islands. The first bird uses 
its large beak to crack open and eat large seeds. Bird #3 is able to pull small seeds out of small spaces. 

(Source: http://en.wikipedia.Org/wiki/lmage:Darwin%27s_finches.jpg) 

In his diary, Darwin pointed out how each animal is well-suited for its particular environment. The shape of 
the finch's beaks on each island were well-matched with the seeds available on their particular island, but 
not the seeds on other islands. A larger and stronger beak was needed to break open large seeds and a 
small beak was needed to feed on some of the smallest seeds. 

Darwin also noticed how different species were distributed around the world. The finch, tortoise and other 
species found on the Galapagos Islands were similar to species on South America, the nearest continent. 



138 



Yet they also differed. Likewise, species he saw on islands near Africa were similar to, but different from 
species on Africa. 

When Darwin returned to England five years later, he did not rush to announce his discoveries. Unlike other 
naturalists before him, Darwin did not want to present any ideas unless he had strong evidence supporting 
them. Instead, once Darwin returned to England, he spent another twenty years examining specimens, 
talking with other scientists and collecting more information before he presented his theories. Darwin's ob- 
servations eventually resulted in the Theory of Evolution by Natural Selection. His now famous book, The 
Origin of Species was a diary of his explorations and discussion on how he interpreted his observations. 




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Figure 10: Charles Darwin presented the Theory of Evolution by Natural Selection in this book. The theories 
were based on evidence he collected and tested. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Origin_of_Species_title_page.jpg, License: Public Domain) 

Other Influences on Darwin 

How did Darwin come up with his theories? Some of Darwin's idea conflicted with widely held beliefs, included 
those from religious leaders, such as: 



• All organisms never change and never go extinct, they are fixed. 

• The world is only about 6,000 years old, based on the stories in the Bible. 

It was because of these widely held beliefs that delayed Darwin from presenting his findings. 
Charles Darwin was influenced by the ideas from several people. 



139 



Before his voyage on the Beagle: 

1 . Jean-Baptiste Lamarck proposed the idea that evolution occurs. However, Darwin differed with Lamarck 
on several other points. Lamarck proposed that traits acquired during one's lifetime could be passed to the 
next generation. 

2. Darwin's grandfather, Erasmus Darwin wrote a book called Zoonomia. Charles Darwin was influenced 
by many of his grandfather's ideas including his descriptions of how species change (evolve) through artificial 
selection. During artificial selection, people choose specific traits to pass to the next generation, such as 
with horse or dog breeding (See below). 

3. Charles Lyell, a well-known geologist and one of Darwin's instructors. Darwin learned about geology, 
paleontology and the changing Earth from Lyell. 

4. Thomas Malthus: Darwin's ideas of natural selection were inspired by reading an essay by Thomas 
Malthus, an economist suggested that humans could overpopulate and potentially exhaust food supplies. 
Darwin thought this must be especially true for animals, as they have a tendency to have more offspring 
than people have. There would therefore be a competition for survival. 

5. Charles Darwin came upon some of his ideas about natural selection and adaptations from reading about 
artificial selection and breeding dogs. All dogs, from Chihuahuas to St. Bernards are part of the same species 
as wolves (canine). Humans created the different breeds of dogs by selecting dogs with specific traits to 
breed together. For example, greyhounds were created by selecting the fastest runners and breeding them 
together. 



■^» ^P* ™ ^" ^^ ^f 



Figure 11 : Darwin's grandfather had a big influence on Darwin's ideas by introducing him to artificial selection 
of dogs and horses. Humans have created hundreds of dog breeds by selecting which dogs to breed based 
on certain features, such as size, coloration, speed, or facial features. 

(Source: http://en.wikipedia.org/wiki/Dog) 

After the Voyage of the Beagle: 

6. Alfred Russel Wallace, another naturalist, also developed a theory of evolution by natural selection. Alfred 
Wallace toured South American and came up with a very similar theory of evolution by natural selection at 
the same time that Darwin did. Darwin and Wallace presented their theories and evidence in public together. 
Because of the vastness of Darwin's data, and his book, he is mostly credited and associated with this theory. 

Natural Selection and Adaptation 

The Theory of Evolution by Natural Selection means that the inherited traits of a population change over 
time through natural selection. Inherited traits are features that are passed from one generation to the next. 
For example, your eye color is an inherited trait (you inherited from your parents). Acquired traits are features 



140 



such as a tan from lying in the sun or strong muscles from working out. 

Natural selection happens when some organisms have traits that make them better suited (they have better 
accommodation) to live in a certain environment than others. They are more likely to survive, reproduce and 
pass their traits on to future generations than those without the special traits. The process of natural selection 
helps us understand how organisms appear to be so well suited or adapted to their environments. Every 
plant and animal depends on its traits to survive. Survival may include getting food, building homes, and 
attracting mates. Most of these traits have been changed through natural selection so they allow a plant, 
animal, or bacteria to survive and reproduce relatively well in their environments. These traits are called 
adaptations. 

Imagine how in winter dark fur makes a rabbit easy for fox to spot and catch in the snow. Natural selection 
suggests that white-fur is an advantageous trait that improves the chance that a rabbit will survive, reproduce 
and pass the trait of white fur on to future generations. Dark fur rabbits will become uncommon. 




Figure 12: In winter, the fur of arctic hares turns white. The camouflage may make it more difficult for fox 
and other predators to locate hares against the white snow. 

(Source: http://en.wikipedia.Org/wiki/lmage:Arctic_Hare.jpg, License: Public Domain) 

Lesson Summary 

Evolution is change in species over multiple generations. 

Natural selection is how evolution occurs. 

Adaptations are the result of natural selection. 

Charles Darwin is credited with developing the Theory of Evolution by Natural Selection 

Darwin collected much of his evidence on a five year voyage around the world, with much of his data 
collected on the Galapagos Islands. 

The work of many others contributed to Darwin's theory. 
Review Questions 

1 . What is biological evolution? 

2. What is natural selection? 

3. What is adaptation? 

4. What is the difference between an inherited trait and an acquired trait? 

5. What was the name of the ship that Darwin traveled on? 



141 



6. What is the name of the islands where Darwin studied evolution? 

7. A giraffe's long neck allows the giraffe to eat leaves from high in the tree, this is an example of an: 

8. Who proposed a theory of evolution by natural selection that was similar to Darwin's theory? 

Further Reading I Supplemental Links 

Stein, Sara, The Evolution Book, Workman, N.Y., 1986. 

Yeh, Jennifer, Modern Synthesis, (From Animal Sciences). 

Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859. 

Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003. 

Ridley, Matt, Genome, HarperCollins, 2000. 

Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006. 

Carroll, Sean B., The Making of the Fittest: DNAand the Ultimate Forensic Record of Evolution, Norton, 
2006. 

Dawkins, Richard, The Blind Watchmaker, W.W. Norton & Company, 1996. 

Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989. 

Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins, 
2006. 

Mayr, Ernst, What Evolution Is, Basic Books, 2001 . 

Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008. 

http://en.wikipedia.org/ 

Vocabulary 



acquired trait 
adaptation 

artificial selection 
evolution 



evolution by natural 
selection 

Galapagos Islands 



inherited traits 



A feature that an organism gets during its lifetime in response to the environment 
(not from genes); not passed on to future generations through genes. 

Beneficial traits that help an organism survive in its environment. Organisms with 
beneficial traits are more likely to survive, reproduce and pass their traits on to 
future generations than those without the special traits. These traits are called 
adaptations. 

Selection in which people choose specific traits to pass to the next generation, 
such as with horse or dog breeding. 

A process in which something passes by degrees to a different stage, such as a 
living organism turning into a more advanced or mature organism; the change of 
the inherited traits of a group of organisms over many generations. 

The changes in the inherited traits of a population from one generation to the next; 
due to a process where organisms that are best suited to their environments have 
greater survival and reproductive success. 

A group of islands in the Pacific off South America; owned by Ecuador; known for 
unusual animal life. Many scientists, including Charles Darwin made many discov- 
eries that led to the theory of evolution by natural selection while studying the 
plants and animals on these islands. 

Features that are passed from one generation to the next. 



142 



natural selection Results when some organisms have traits that make them better suited to live in 

a certain environment than others; they are more likely to survive, reproduce and 
pass their traits on to future generations than those without the special traits. 

species A group of individuals that are genetically related and can breed to produce fertile 

young. 

trait A feature or characteristic of an organism. For example, your height, hair color, 

and eye shape are physical traits. 

Review Answers 

1 . The change of an inherited trait in a population over many generations, such as the change of the color 
of moths living on an island over many generations. 

2. Individuals that are best suited to their environment are more likely to survive, reproduce and pass needed 
(desirable) traits (characteristics) on to their offspring. 

3. A trait that allows an organisms to live in a specific environment. 

4. Inherited traits are passed on from generation to generation; acquired traits are not passed on. 

5. The HMS Beagle 

6. The Galapagos Islands 

7. Adaptation 

8. Alfred Russel Wallace 

Points to Consider 

• Evolution by natural selection is supported by extensive scientific evidence. 



Evidence of Evolution 

Lesson Objectives 

• understand the scientific theory of biological evolution is based on extensive physical evidence and 
testing. This includes: 



differences between fossils in different layers of rock 



the age of rocks and fossils 



vestigial structures 



similarities between embryos of different organisms 



143 



the same DNA and RNA materials found in all organisms 



• similar genomes found in almost all organisms 

• Know that creationism is not a scientific theory because science cannot be used to test nonphysical ev- 
idence such as the existence of supernatural entities. 

Check Your Understanding 

• Where did Charles Darwin collect evidence of evolution and what kinds of evidence did he find? 

• What is natural selection? 

• What kinds of traits change through evolution? 

Introduction 

Though the idea of evolution had been proposed prior to Charles Darwin, most people think of Darwin's 
name when they think of evolution. Unlike others before him who based their ideas on speculation, opinions, 
myths, or folklore, Darwin's theories were based on a tremendous amount of scientific evidence. 

In 1 859, Charles Darwin and Alfred Russel Wallace first presented several forms of evidence of evolution. 
Their evidence included: 

fossils of extinct species from different eras 

similarities between the embryos of different species 

physical traits of different species 

the behavior of different species 

the distributions of different plant and animal species around the world. 

Darwin and other 19 th century scientists came to the conclusions they did without knowing anything about 
molecular biology. Today, even more evidence of evolution by natural selection is coming from genetics. 
Genetics is also helping explain the mechanisms of how evolution occurs. 

The Fossil Record 

Paleontologists are the scientists who study fossils to learn about life in the past. Fossils are the preserved 
remains or traces of animals, plants, and other organisms from the distant past. Examples of fossils include 
bones, teeth, impressions, and leaves. Paleontologists compare the features of species from different periods 
in history. With this information, they try to unravel how species have evolved over millions of years (Figure 
1 ). This method works better with some species than others. For example, it is difficult to track the evolution 
of bacteria from fossils, because their single cells do not last well as fossils. 



144 




EQhiJXWB 



6ft moan ygarj i>J3 



Figure 1: Evolution of the horse. Fossil evidence, depicted by the skeletal fragments, demonstrates evolu- 
tionary milestones in this process. 

(Source: http://www.bringyou.to/apologetics/HorseEvolution.gif) 

Until recently, fossils were the main source of evidence of evolution (Figures 2, 3). The location of each 
fossil in layers of rocks provides clues to the age of the species and how species evolved in the past. Older 
materials and fossils are deeper in the earth; newer fossils and materials are closer to the surface. 



145 




Figure 2: A fossil is the remains of a plant or animal that existed some time in the distant past. Fossils, such 
as this one, were found in rocks or soil that was laid down long ago. 

(Source: http://en.wikipedia.org/wiki/Fossil, License: Public Domain) 




Figure 3: About 40 to 60 million years ago this mosquito and fly were trapped in the gooey stuff, called resin 
that comes from trees. The fossils in the movie Jurassic Park, were trapped in resin. 

(Source: http://en.wikipedia.Org/wiki/lmage:lnsects_in_baltic_amber.jpg, License: GNU Free Documentation) 

Fossils and the rocks they are embedded in provide evidence of how life and environmental conditions have 
changed throughout earth's history. They also help us understand how the past and present distribution of 
life on earth is affected by earthquakes, volcanoes, and shifting seas, and other movements of the continents. 

The Age of Rock Layers and Fossils 

The many layers of sedimentary rock provide evidence of the long history of earth and the order of life forms 
whose remains are found in the rocks. The youngest layers are not always found on top, because of folding, 
breaking, and uplifting of layers. If the layers of earth were tilted by earthquakes or volcanoes, geologists 
can figure out which layers came from the deepest parts of the earth. 

The fossils and the order in which fossils appear is called the fossil record. This record provides important 
records of how species have evolved, divided and gone extinct. Methods used to date the age of rocks and 
fossils make it possible to determine when these events happened. 

Geologist use a method called radiometric dating to determine the age of rocks and fossils in each layer of 
rock. This technique measures the decay rate of radioactive materials in each rock layer (Figure 4). 



146 




Figure 4: This device, called a spectrophotometer can be used to measure the level of radioactive decay 
of certain elements in rocks and fossils to determine their age. 

(Source: http://en.wikipedia.0rg/wiki/lmage:ls0t0pe_rati0_ms.jpg, License: Public Domain) 

Radiometric dating has been used to determine that the oldest known rocks on earth are between 4-5 billion 
years old. The oldest fossils are between 3-4 billion years old. 

Vestigial Structures 

Millions of species of animals, plants and microorganisms are alive today. Even though two different species 
may not look similar, they may have similar internal structures, and chemical processes that indicate they 
can have a recent common ancestor. 

Some of the most interesting kinds of evidence for evolution are body parts that have lost their use through 
evolution (Figure 5). Most birds need their wings to fly. But the wings on an ostrich have lost their original 
use. These are called vestigial structures. Penguins do not use their wings to fly in the air; however they 
do use them to "fly" in the water. A whale's pelvic bones are also vestigial structures (Figure 6). 







■S^tOi^ -^iSSS 



Figure 5: Mole rats live under ground where they do not need eyes to find their way around. This mole's 
eyes are covered by skin. Body parts that do not serve any function are called vestigial structures. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Blindmaus-drawing.jpg, License: Public Domain) 



147 




Figure 6: The bones in your arms and hands have the same bone pattern as those in the wings, legs, and 
feet of the mammals pictured above. How have the bones adapted for different uses in each animal? 

(Source: http://upload.wikimedia.Org/wikipedia/commons/thumb/5/5b/Evolution_pl.png/500px-Evolution_pl.png) 

If you look at an x-ray of the bones in your back (called vertebrae), you will see several vertebrae that come 
under your hips. These are called your tailbone. We do not use these small vertebrae. Yet they are further 
evidence of our evolution. 

Embryological Evidence 

Some of the oldest evidence of evolution comes from embryology, the study of how organisms develop. An 
embryo is an animal or plant in its earliest stages of development, before it is born or hatched. 

Centuries ago, people recognized that the embryos of many different species have similar appearances 
(Figure 7). The embryos of some species are even difficult to tell apart. Many of these animals do not differ 
much in appearance until they develop further. Many traits of one type of animal appear in the embryo of 
another type of animal. For example, fish embryos and human embryos both have gill slits. In fish they develop 
into gills, but in humans they disappear before birth (Figure 8). 

The similarities between embryos suggests that these animals are related and have common ancestors. 
For example, humans did not evolve from chimpanzees. But the similarities between the embryos of both 
species may be due to our development from a common ancestor with chimpanzees. As our common an- 
cestor evolved, both humans and chimpanzees developed different traits. 



148 




Fish Salamander Tortoise Chick Hog Calf Rabbit Human 



Figure 7: This drawing was made to show the similarities between the embryos of many species. Embryos 
of many different kinds of animals: mammals, birds, reptiles, fish, etc. look very similar. 

(Source: http://en.wikipedia.Org/wiki/lmage:Haeckel_drawings.jpg, License: Public Domain) 




Figure 8: This is a six week old human embryo. Notice the similarities between this embryo and those of 
the other animals in figure 3. 

(Source: http://en.wikipedia.org/wiki/Embryology, License: CC-BY-SA) 
Similarities Between Molecules and Genomes 

Arguably, some of the most significant evidence of evolution comes from examining the molecules and DNA 
found in all organisms (Figure 9). The field of molecular biology did not emerge until the 1940s and has 
since confirmed and extended the conclusions about evolution drawn from other forms of evidence. 
Molecular clocks are used in molecular evolution to relate the time that two species diverged to the number 
of differences measured between the species' DNA sequences or protein amino acid sequences. These 
clocks are sometimes called gene clocks or evolutionary clocks. Molecular clocks, combined with other 
forms of evidence, such as evidence from the fossil record, has provided considerable evidence to estimate 
how long ago various groups of organisms diverged evolutionary from one another. 



149 




Figure 9: Almost all organisms are made from DNA with the same building blocks. The genomes (all of the 
genes in an organism) of all mammals are almost identical 

(Source: http://en.wikipedia.0rg/wiki/lmage:DNA_Overview.png) 

The development of molecular genetics has revealed the record of evolution left in the genomes of all organ- 
isms (Figure 1 0). It also provides new information about the relationships among species and how evolution 
occurs. 

Molecular genetics provides evidence of evolution such as: 

• the same biochemical building blocks - such as amino acids and nucleotides - are responsible for life 
in all organisms, from bacteria to plants and animals 

• DNA and RNA determines the development of all organisms 

• the similarities and differences between the genomes, the gene sequences of each species, reveal patterns 
of evolution. 



150 



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Figure 10: This is a map of the genes on just one of the 46 human chromosomes. Similarities and differences 
between the genomes (the genetic makeup) of different organisms reveal the relationships between the 
species. The human and chimpanzee genomes are almost identical. The complexity of the map signifies 
close evolutionary relationships when the genomes are highly similar. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Gen0me_viewer_screensh0t_small.png, License: Public Domain) 

Lesson Summary 

Fossil evidence, depicted by the skeletal fragments, demonstrates evolutionary milestones. 

Fossils and the rocks they are embedded in provide evidence of how life and environmental conditions 
have changed throughout earth's history. 

The fossils and the order in which fossils appear is called the fossil record. 

Geologist use a method called radiometric dating to determine the age of rocks and fossils in each layer 
of rock. 

Radiometric dating has been used to determine that the oldest known rocks on earth are between 4-5 
billion years old. The oldest fossils are between 3-4 billion years old. 

Body parts that do not serve any function are called vestigial structures. 

Vestigial structures indicate that two species have a recent common ancestor. 



151 



• The similarities between embryos suggests that animals are related and have common ancestors. 

• The same biochemical building blocks - such as amino acids and nucleotides - are responsible for life 
in all organisms, from bacteria to plants and animals. 

• DNA and RNA determines the development of all organisms. 

• The similarities and differences between the genomes, the gene sequences of each species, reveal 
patterns of evolution. 

Review Questions 

1 . What are the different kinds of evidence of evolution? 

2. How do geologists determine the age of rocks and fossils? 

3. What is an embryo? 

4. What is a vestigial structure? 

5. What is an example of a vestigial structure? 

6. What is a genome? 

7. What is the most convincing evidence of evolution? 

8. How do the embryos of different species support the idea of evolution? 

Further Reading I Supplemental Links 

Stein, Sara, The Evolution Book, Workman, N.Y., 1986. 

Yeh, Jennifer, Modern Synthesis, (From Animal Sciences). 

Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859 . 

Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature. Perennial Books, 2003. 

Ridley, Matt, Genome, HarperCollins, 2000. 

Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006. 

Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton, 
2006. 

Dawkins, Richard, The Blind Watchmaker, W.W. Norton & Company, 1996. 

Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989. 

Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins, 
2006. 

Mayr, Ernst, What Evolution Is, Basic Books, 2001 . 

Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008. 



152 



• http://en.wikipedia.org/ 
Vocabulary 

embryo An animal or plant in its earliest stages of development, before it is born or hatched. 

embryology The study of how organisms develop. 

fossil The preserved remains or traces of animals, plants, and other organisms from the 

distant past; examples include bones, teeth, impressions, and leaves. 

fossil record Fossils and the order in which fossils appear; provides important records of how 

species have evolved, divided and gone extinct. 

genetics The scientific study of heredity. 

genome All of the genes in an organism. 

paleontologists Scientists who study fossils to learn about life in the past. 

radiometric dating A method to determine the age of rocks and fossils in each layer of rock; measures 
the decay rate of radioactive materials in each rock layer. 

vestigial structure Body part that has lost its use through evolution, such as a whale's pelvic bones. 

Review Answers 

1 . Fossils that show the evolution of past species, the age of rocks and fossils, similarities and differences 
between the molecules and genes of different species, similarities between embryos of different species 

2. By measuring the radioactive decay of minerals in certain rocks. 

3. An early stage of development. 

4. A body part that has lost its function through evolution. 

5. The eyes of a mole rat, and other potential answers. 

6. The genetic make up of an organism. 

7. Similarities between the genomes of different species. 

8. Similarities between embryos suggest that the species are related and have common ancestors. 

Points to Consider 

• How do you think new species evolve? 

• How long do you think it takes for a new species to evolve? 

Macroevolution 

Lesson Objectives 

• Students will understand the differences between macroevolution and microevolution. 

• Students will understand that speciation is the formation of new species. 



153 



• Students will understand the mechanisms of speciation. 

Check Your Understanding 

• Why can't an individual person evolve? Why can only groups evolve over many generations? 

• What causes a species or a population to evolve? 

Introduction 

Small changes or large changes, how does evolution occur? It is easy to think that many small changes, as 
they accumulate over time, may gradually lead to a new species. Or is it possible that due to severe changes 
in the environment, large changes are needed to allow species to adapt to the new surroundings? Or are 
both probable methods of evolution? 

Microevolution and Macroevolution 

You already know that evolution is the change in species over time, due to the change of how often an in- 
herited trait occurs in a population over many generations. Most evolutionary changes are small and do not 
lead to the creation of a new species. These small changes are called microevolution. 

An example of microevolution is the evolution of pesticide resistance in mosquitoes. Imagine that you have 
a pesticide that kills most of the mosquitoes in your state one year. As a result, the only remaining mosquitoes 
are the pesticide resistant mosquitoes. When these mosquitoes reproduce the next year, they produce more 
mosquitoes with the pesticide resistant trait. This is an example of microevolution because the number of 
mosquitoes with this trait changed. However, this evolutionary change did not create a new species of 
mosquito, because the pesticide resistant mosquitoes can still reproduce with other mosquitoes if they were 
put together. 

Macroevolution refers to much bigger evolutionary changes that result in new species. Macroevolution 
may happen: 

1 . when many microevolution steps lead to the creation of a new species, 

2. as a result of a major environmental change, such as volcanic eruptions, earthquakes or an asteroid 
hitting earth, which changes the environment so much that natural selection leads to large changes in the 
traits of a species 

After thousands of years of isolation from each other, some of Darwin's finch population, which was discussed 
in the Evolution by Natural Selection lesson, will not or cannot breed with other finch populations when they 
are brought together. Since they do not breed together, they are classified as separate species. 

Hardy-Weinberg Equilibrium 

The Hardy-Weinberg model (sometimes called a law) states that a population will remain at genetic equilibrium 
- with constant allele and genotype frequencies and no evolution - as long as five conditions are met: 

1. No mutation (no change in the DNA sequence) 

2. No migration (no moving into or out of a population) 

3. Very large population size 

4. Random mating (mating not based on preference) 

5. No natural selection 

These five conditions rarely occur in nature. For example, it is highly unlikely that new mutations are not 
constantly generated. 



154 



If these five conditions are met, the frequencies of genotypes within a population can be determined given 
the phenotypic frequencies. For example, let's use a hypothetical rabbit population of 1 00 rabbits (200 alleles) 
to determine allele frequencies: 

• 9 albino rabbits (represented by bb) and 

• 91 brown rabbits (49 homozygous [BB] and 42 heterozygous [Bb]). 

The gene pool contains 140 B alleles (70%) and 60 b alleles (30%) - which have gene frequencies of 0.7 
and 0.3, respectively. 

If we assume that alleles sort independently and segregate randomly as sperm and eggs form, and that 
mating and fertilization are also random, the probability that an offspring will receive a particular allele from 
the gene pool is identical to the frequency of that allele in the population: 

• BB: 0.7x0.7 = 0.49 

• Bb: 0.7x0.3 = 0.21 

• bB: 0.3x0.7 = 0.21 

• bb: 0.3x0.3 = 0.09 

If we calculate the frequency of genotypes among the offspring, they are identical to the genotype frequencies 
of the parents. There are 9% bb albino rabbits and 91% BB abd Bb brown rabbits. Allele frequency remains 
constant as well. The population is stable - at a Hardy-Weinberg genetic equilibrium. 

A useful equation generalizes the calculations we've just completed. Variables include 

• p = the frequency of one allele (we'll use allele B here) and 

• q = the frequency of the second allele ( b , in this example). 

We will use only two alleles (so p + q must equal 1 ), but similar equations can be written for more alleles. 

Allele frequency equals the chance of any particular gamete receiving that allele. Therefore, when egg and 
sperm combine, the probability of any genotype is the product of the probabilities of the alleles in that 
genotype. So: 

Probability of genotype BB = p X p = p 2 and 

Probability of genotype Bb = ( p X q) + (q X p) = 2pq and 

Probability of genotype bb = ijX <7=q 2 

We have included all possible genotypes, so the probabilities must add to 1 .0. Our equation becomes: 

2 +2 pq + n 2 =1 

frequency of genotype BB frequency of genotype Bb frequency of genotype bb 

In our example: 0.49 + 2(0.21 ) + 0.9 = 1 . 

This is the Hardy-Weinberg equation, which describes the relationship between allele frequencies and 
genotype frequencies for a population at equilibrium. 

Recall that the third requirement for Hardy-Weinberg equilibrium is a very large population size. This is be- 
cause variations in allele frequencies that occur by chance are minimal in large populations. In small popu- 



155 



lations, random variations in allele frequencies can significantly influence the "survival" of any allele. Random 
changes in allele frequencies in small populations are known as genetic drift. As the population (and 
therefore the gene pool) is small, genetic drift could have substantial effects on the traits and diversity of a 
population. Many biologists think that genetic drift is a major cause of microevolution. 

The Origin of Species 

The creation of a new species is called speciation. Most new species develop naturally, but humans have 
also artificially created new subspecies, breeds, and species for thousands of years. 

Natural selection causes beneficial heritable traits to become more common in a population, and unfavorable 
heritable traits become less common. For example, a giraffe's neck is beneficial because it allows the giraffe 
to reach leaves high in trees. Natural selection caused this beneficial trait to become more common than 
short necks. 

As new mutations (changes in the DNA sequence) are constantly being generated in a population's gene 
pool, some of these mutations will be beneficial and result in traits that allow adaptation and survival. Natural 
selection causes evolution through the genetic change of a species as the beneficial traits become more 
common within a population. 

Artificial selection is when humans select which plants or animals to breed to pass specific traits on to the 
next generation. A farmer may choose to breed only the cows that produce the best milk (the favored traits) 
and not breed cows that do not produce much milk (a less desirable trait). Humans have also artificially 
breed dogs to create new breeds (Figure 1 ). 




Figure 1: Artificial Selection: Humans used artificial selection to create these different breeds. Both dog 
breeds are descended from the same wolves, and their genes are almost identical. Yet there is at least one 
difference between their genes that determine size. 

(Source: en.wikipedia.org/wiki/Artificial_selection, License: GNU Free Documentation) 

There are two main ways that speciation happens naturally. Both processes create new species by isolating 
groups (populations) of the same species from each other. Organisms can be reproductively isolated from 
each other either geographically or by some behavior. Over long period of time (usually thousands of years), 
each population evolves in a different direction. One way scientists test whether two populations are separate 
species is to bring them together again. If the two populations do not interbreed and produce fertile offspring, 
they are separate species 

Geographic Isolation 

Allopatric speciation happens when groups from the same species are geographically isolated physically 
for long periods. Imagine all the ways that plants or animals could be isolated from each other: 



a mountain range 



156 



a canyon water such as rivers, streams, or an ocean 



• a desert 

Charles Darwin recognized that speciation could happen when some members of a species were isolated 
from the others for hundreds or thousands of years. Darwin had observed thirteen distinct finch species on 
the Galapagos Islands that had evolved from the same ancestor. Several of the finch population evolved 
into separate species while they were isolated on separate islands. Scientists were able to determine which 
finches had evolved into distinct species by bringing members of each population together. The birds that 
would not or could not interbreed are regarded as separate species. 

A classic example of geographic isolation is the Abert squirrel, shown in Figures 2 and 3. When the Grand 
Canyon in Arizona formed, squirrels from one species were separated by the giant canyon that they could 
not cross. After thousands of years of isolation from each other, the squirrel populations on the northern 
wall of the canyon looked and behaved differently from those on the southern wall. North rim squirrels have 
white tails and black bellies. Squirrels on the south rim have white bellies and dark tails. 




Figure 2: Aberts Squirrel on the southern rim of the Grand Canyon 

(Source: http://en.wikipedia.Org/wiki/lmage:SCIURUS_ABERTI.jpg, License: Public Domain) 




Figure 3: Kaibab squirrel (a subspecies of Abert's) found on northern rim of the Grand Canyon 



157 



(Source: http://en.wikipedia.0rg/wiki/File:Stavenn_Sciurus_aberti_kaibabensis_OO.jpg, License: GNU Free 
Documentation) 

Isolation without Physical Separation 

Sympatric speciation happens when groups from the same species stop interbreeding, because of 
something other than physical separation, such as behavior. The separation may be due to different mating 
seasons, for example. Sympatric speciation is more difficult to identify. 

Some scientists suspect that two groups of orcas (killer whales) live in the same part of the Pacific Ocean 
part of the year, but do not interbreed. The two groups hunt different prey species, eat different foods, sing 
different songs, and have different social structures. Different behaviors may have led to the emergence of 
two Galapagos finch species that live in the same space. The two species are separated by behavioral 
barriers such as mating signals. In this case, members of each group select mates according to different 
beak structures and bird calls. They do not need physical barriers, because behavioral differences do enough 
to keep the groups separated. 

Allopatric speciation and sympatric speciation are both forms of reproductive isolation. Allopatric speciation 
is due to geographic isolation. Sympatric speciation is due to behavioral isolation, or isolation due to 
different mating seasons, which is also known as temporal isolation. 

Rates of Evolution 

How fast is evolution? How long did it take for the giraffe to develop a long neck? How long did it take for 
the Galapagos finches to evolve? How long did it take for whales to evolve from land mammals? These and 
other questions about the rate of evolution are difficult to answer, but evidence does exist in the fossil record. 

The rate of evolution is a measurement of the speed of evolution. Genetically speaking, evolution is how 
much an organism's genotype (the genes that make up an individual) changes over a set period of time. 
Evolution is usually so gradual that we do not see the change for many, many generations. Humans took 
millions of years to evolve from a mammal that is extinct now. 

Not all organisms evolve at the same rate. It would be difficult to measure evolution on your family because 
you are only looking at a small population over a few generations. However there are organisms that are 
evolving so fast that you may be able to observe evolution! Many scientists use bacteria or other species 
that reproduce frequently to study evolution. Species with short life cycles and that reproduce frequently 
evolve much faster than others. Bacteria evolve hundreds (or thousands or more) of times faster than humans 
do. Bacteria go through so many generations in a few days, that we can actually witness evolution. A human 
takes about 22 years to go through one generation. But some bacteria go through over a thousand generations 
in less than two months. 

Evolutionary Trees 

Charles Darwin came up with the idea of an evolutionary tree to represent the relationships between different 
species and their common ancestors (Figure 4). The base of the tree represents the ancient ancestors of 
all life. The separation into large branches shows where these original species evolved into increasingly 
different populations that would not come back together again. The branches keep splitting into smaller and 
smaller branches as species continue to split into more and more species. Some species are represented 
by short twigs spurting out of the tree, then stopping. These are species that went extinct before evolving 
into new species. Other "Trees of Life" have been created by other scientists (Figure 5). 



158 



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(Source: http://en.wikipedia.Org/wiki/lmage:Darwins_tree_of_life_1859.gif, License: Public Domain) 



159 



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Figure 5: Scientists have drawn many different versions of the "Tree of Life" to show different features of 
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(Source: http://en.wikipedia.0rg/wiki/lmage:Tree_0f_life_by_Haeckel.jpg, License: Public Domain) 

Lesson Summary 

• Microevolution results from evolutionary changes that are small and do not lead to the creation of a new 
species. 

• Macroevolution refers to large evolutionary changes that result in new species. 

• Macroevolution may happen when many microevolution steps lead to the creation of a new species. 

• Macroevolution may happen as a result of a major environmental change, such as volcanic eruptions, 
earthquakes or an asteroid hitting earth, which changes the environment so much that natural selection 
leads to large changes in the traits of a species 

• The creation of a new species is called speciation. 



160 



Natural selection causes beneficial heritable traits to become more common in a population, and unfa- 
vorable heritable traits become less common. 

Artificial selection is when humans select which plants or animals to breed to pass specific traits on to 
the next generation. 

Allopatric speciation occurs when groups from the same species are geographically isolated physically 
for long periods. 

Sympatric speciation occurs when groups from the same species stop interbreeding, because of something 
other than physical separation, such as behavior. 

Allopatric speciation and sympatric speciation are both forms of reproductive isolation. 

The rate of evolution is a measurement of the speed of evolution. Genetically speaking, evolution is how 
much an organism's genotype changes over a set period of time. 

Not all organisms evolve at the same rate. 

Evolutionary trees are used to represent the relationships between different species and their common 
ancestors. 

Review Questions 

1. What is the difference between macroevolution and microevolution? 

2. What conditions cause organisms to evolve and adapt? 

3. What do the branches on the Tree of Life represent? 

4. Which organism has a faster rate of evolution: a human or a bacterium? 

5. How do you know if two related organisms are members of the same species? 

6. Why do the squirrels on opposite side of the Grand Canyon look different? 

7. How is artificial selection different from natural selection? 

8. What, other than physical isolation, could cause a species to split into two different directions of evolution? 

Further Reading I Supplemental Links 

Yeh, Jennifer, Modern Synthesi, (From Animal Sciences). 

Darwin, Charles, Origin of the Specie, Broadview Press (Sixth Edition), 1859. 

Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003. 

Ridley, Matt, Genome, HarperCollins, 2000. 

Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006. 

Carroll, Sean B., The Making of the Fittest: DNAand the Ultimate Forensic Record of Evolution, Norton, 
2006. 

Dawkins, Richard, The Blind Watchmaker, W.W. Norton & Company, 1996. 

Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989. 

Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins, 
2006. 



161 



• Mayr, Ernst, What Evolution Is, Basic Books. 2001 . 

• Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008. 

• http://en.wikipedia.org/ 

Vocabulary 



allopatric speciation 

artificial selection 

behavioral isolation 

evolutionary tree 

genotype 
geographic isolation 

macroevolution 
microevolution 
natural selection 

primate 

reproductive isolation 

speciation 
sympatric speciation 

temporal isolation 
Review Answers 



Speciation that occurs when groups from the same species are geographically 
isolated physically for long periods. 

Occurs when humans select which plants or animals to breed to pass specific 
traits on to the next generation. 

The separation of a population from the rest of its species due to some behavioral 
barrier, such as having different mating seasons. 

Diagram used to represent the relationships between different species and their 
common ancestors. 

The genes that make up an individual. 

The separation of a population from the rest of its species due to some physical 
barrier, such as a mountain range, an ocean, or great distance. 

Big evolutionary changes that result in new species. 

Small changes in inherited traits; does not lead to the creation of a new species. 

Causes beneficial heritable traits to become more common in a population, and 
unfavorable heritable traits become less common. 

A group of related mammal species that have binocular vision, specialized hands 
and feet for grasping, and enlarged and differentiated brains; includes humans, 
chimpanzees, the apes, monkeys, and lemurs. 

allopatric and sympatric speciation; isolation due to geography or behavior, resulting 
in the inability to reproduce. 

The creation of a new species; either by natural or artificial selection. 

Speciation that occurs when groups from the same species stop interbreeding, 
because of something other than physical separation, such as behavior. 

Isolation due to different mating seasons. 



1 . Microevolution is the small evolutionary change that is always happening. Macroevolution is large evolu- 
tionary change that results in the creation of a new species 

2. Changes of environmental conditions and isolation from each other 

3. Organisms evolving in different directions 

4. the little bacterium 

5. If they can interbreed and produce fertile offspring, they are members of the same species. 

6. They were isolated from each other for thousands of years and developed some different traits 

7. Humans select how a plant or animals change instead of nature. 

8. Different kinds of behavior or choices of food 

Points to Consider 

• How long do you think humans have been around? 



162 



• How long do you think Earth existed before life formed? 

• For how much of Earth's history have humans existed? 

History of Life on Earth 

Lesson Objectives 

• Know that geologists and paleontologists use evidence to determine the history of Earth and life on earth. 

• Know that geologists and paleontologists measure the radioactivity in certain rocks to determine the age 
of the earth and fossils. 

• Know that the earth is between four and five billion years old. 

• Know that scientists need to know what the environment (what chemicals were around, the temperature, 
etc.) was like on earth billions of years ago to know how life formed. 

Check Your Understanding 

• What are fossils? 

• How does the fossil record contribute to the evidence of evolution? 

Introduction 

It is no surprise that people have wondered about the age of the earth, how it was formed, and how life began 
on earth for hundreds, even thousands, of years. Try to imagine how ancient philosophers tried to explain 
the history of the earth and life. Many people used mythology or cultural beliefs to explain elaborate stories 
about how and when the earth formed. 

The past two to three hundred years has been an exciting time for geologists, paleontologists and other 
scientists who are trying to trace the history of the earth. What was once a hobby, studying land forms and 
fossils has become a science that is revealing the history of the earth and life on earth. 

Age of Earth 

During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence that 
confirmed that the earth is very old, far older than the 6,000 years that some religious and scientific leaders 
claimed. Their evidence included: 

• Fossils of ancient sea life on dry land far from oceans: This supported the idea that the earth changed 
over time and that some dry land today was once covered by oceans. 

• The many layers of rock: When people realized that rock layers represent the order in which rocks and 
fossils appeared, they were able to start to trace the history of the earth and life on earth. 

• Indications that volcanic eruptions, earthquakes and erosion that happened long ago shaped much of 
the earth's surface. This supported the idea of an old earth. 

During the past one hundred years, geologists and paleontologists have been able to delve even deeper 
into the earth's history with new tools of science. The most convincing method, called radiometric dating, 
was developed more than one hundred years ago. Scientists found that they could measure the age of rocks 
by measuring the radioactivity of certain minerals in rocks. Rocks are made up of minerals. Geologists and 
paleontologists still use variations of radiometric dating to determine the age of fossils and rocks today 
(Figure 1). 



163 




Figure 1 : The most reliable way to figure out the earth's age is to measure the radioactivity of certain min- 
erals found in rocks (called radiometric dating). This mass spectrophotometer can also be used to measure 
age of fossils from the level of radiation in minerals surrounding the fossil. 

(Source: http://en.wikipedia.0rg/wiki/lmage:ls0t0pe_rati0_ms.jpg, License: Public Domain) 

The earth is at least as old as its oldest rocks. The oldest rock minerals found on earth so far are zircon 
crystals that are at least 4.404 billion years old. These tiny crystals were found in the Jack Hills of Western 
Australia. Since the earth is at least as old as the oldest minerals found on Earth, geologists estimate that 
the minimum age of the earth is 4.404 billion years. 

Likewise, the earth cannot be any older than the solar system. The oldest possible age of the earth is 4.57 
billion years old, the age of the solar system. Geologists and geophysicists based the age of the universe 
on the age of materials within meteorites that are formed within the solar system. 

Origin of Life on Earth 

There is good evidence that life has probably existed on earth for most of earth's history. Some of the oldest 
fossils of life forms on earth are at least 3.5 billion year old fossils of blue green algae found in Australia 
(Figure 2). 




Figure 2: Some of the oldest fossils on earth are stromolites, made of algae and a kind of bacteria, found 
along the coast of Australia. 

(Source: http://www.fas.org/irp/imint/docs/rst/Sect20/A12.html) 



164 



The next step is to determine exactly how life formed billions of years ago. First, scientists need to know 
what the environment was like 3.5 to 4 billion years ago; they need to know what kinds of materials were 
available then that could have been involved in the creation of life. Scientists believe the early earth contained 
no oxygen gas, but did contain other gases, including nitrogen, carbon dioxide, carbon monoxide, water 
vapor, hydrogen sulfide and probably a few others. 

Today, we have evidence that life on earth came from random reactions between chemical compounds that 
formed molecules; in a series of random steps, these molecules created proteins and nucleic acids (RNA 
or DNA), and then cells. We know that the ingredients for life (the building blocks of life), were present at 
the beginning of earth's history. Some chemicals were in water and volcanic gases. Other chemicals would 
have come from meteorites in space. Energy to drive chemical reactions was provided by volcanic eruptions 
and lightening. Keep in mind that this process may have taken as much as 1 billion years. Our understanding 
of how life originated on earth is developing gradually. 




Figure 3: Some clues to the origins of life on earth come from studying the early life forms that developed 
in hot springs, such as the Grand Prismatic Spring at Yellowstone National Park. This spring is approximately 
250 feet by 300 feet wide. 

(Source: http://en.wikipedia.Org/wiki/lmage:Grand_prismatic_spring.jpg) 
Geologic Time Scale 

Geologists and other earth scientists use geologic time scales to describe when events occurred throughout 
the history of earth. The time scales can be used to illustrate when both geologic events and events affecting 
plant and animal life occurred. All of the earth events we see happening today, such as earthquakes, volcanic 
eruptions, and erosion, have happened throughout history. Past catastrophic events, such as asteroids and 
comets also hit the earth long before humans evolved. 

The geologic time scale in figure 4 illustrates the timing of events such as: 



• earthquakes 



volcanoes 



major erosion 



165 



meteorites hitting earth 



the first signs of life forms 



mass exterminations 




Figure 4: The geological time scale of earth's past is organized according to events which took place during 
different periods on the time scale. Geologic time is the same as the age of the earth: between 4.04 and 
4.57 billion years. Look closely for such events as the extinction of dinosaurs and many marine animals. 

(Source: http://en.wikipedia.Org/wiki/lmage:Geologica_time_USGS.png, License: Public Domain) 

Evolution of Major Life Forms 

Life on earth began about 3.5 to 4 billion years ago. The first life forms were single cell organisms, 
prokaryotic organisms, similar to bacteria. The first multicellular organisms did not appear until about 610 
million years ago in the oceans. These of course would be eukaryotic organisms. Some of the first multicel- 
lular forms included sponges, brown algae, and slime molds. 

Many of the modern types of organisms we know today evolved during the next ten million years in an event 
called the Cambrian explosion. This sudden burst of evolution may have been triggered by some environ- 
mental changes that made the environment more suitable for a wider variety of life forms. 

Plants and fungi did not appear until roughly 500 million years ago. They were soon followed by arthropods 
(insects and spiders). Next came the amphibians about 300 million years ago, followed by mammals around 
200 million years ago and birds around 100 million years ago. 

Even though large life forms have been very successful on earth, most of the life forms on earth today are 
still prokaryotes - small, single celled organisms. Fossils indicate that many organisms that lived long ago 



166 



are extinct. Extinction of species is common; in fact, it is estimated that 99% of the species that have lived 
on the earth no longer exist. 

The basic timeline of Earth is a 4.6 billion year old Earth, with (very approximately): 



about 3.5 - 3.8 billion years of simple cells (prokaryotes) 



3 billion years of photosynthesis 



2 billion years of complex cells (eukaryotes) 



1 billion years of multicellular life 



600 million years of simple animals 



570 million years of arthropods (ancestors of insects, arachnids and crustaceans) 



550 million years of complex animals 



500 million years offish and proto-amphibians 



475 million years of land plants 



400 million years of insects and seeds 



360 million years of amphibians 



300 million years of reptiles 



200 million years of mammals 



167 



• 150 million years of birds 

• 130 million years of flowers 

• 65 million years since the non-avian dinosaurs died out 

• 2.5 million years since the appearance of Homo 

• 200,000 years since humans started looking like they do today 

• 25,000 years since Neanderthals died out 

Mass Extinctions 

Extinctions are part of natural selection. Species often go extinct when their environment changes and they 
do not have the traits they need to survive. Only those individuals with the traits needed to live in a changed 
environment survive (Figure 5). 




Figure 5: Humans have caused many extinctions by introducing species to new places. For example, many 
of New Zealand's birds have adapted to nesting on the ground. This was possible because there were no 
land mammals in New Zealand until Europeans arrived and brought cats, fox and other predators with them. 
Several of New Zealand's ground nesting birds, such as this flightless kiwi, are now extinct or threatened 
because of these predators. 

(Source: http://en.wikipedia.Org/wiki/lmage:Apteryx_owenii_0.jpg, License: Public Domain) 

Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major 
catastrophes such as volcanic eruptions and major earthquakes changed the environment. Scientists have 
been looking for evidence of why dinosaurs went extinct over fairly short periods. Many scientists are exam- 
ining the theory that a major cataclysmic events, such as an asteroid colliding with earth, may have caused 
the extinction of dinosaurs 65 million years ago. 



168 




Figure 6: The fossil of Tarbosaurus, one of the land dinosaurs that went extinct during one of the mass 
extinctions. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Tarbosaurus_museum_Muenster.jpg, License: GNU Free Doc- 
umentation) 

Since life began on earth, there have been several major mass extinctions. If you look closely at the geolog- 
ical time scale, you will find that at least five major massive extinctions have occurred in the past 540 million 
years. In each mass extinction, over 50% of animal species died. The total number of extinctions could be 
as high as 20 mass extinctions during this period. 

Lesson Summary 

During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence 
that confirmed that the earth is very old. 

Fossils of ancient sea life on dry land far from oceans supported the idea that the earth changed over 
time and that some dry land today was once covered by oceans. 

The many layers of rock represent the order in which rocks and fossils appeared. 

Indications that volcanic eruptions, earthquakes and erosion that happened long ago shaped much of 
the earth's surface. 

Radiometric dating allows scientists to measure the age of rocks by measuring the radioactivity of certain 
minerals in rocks. 

The oldest rock minerals found on earth so far are zircon crystals that are at least 4.404 billion years old. 

Some of the oldest fossils of life forms on earth are at least 3.5 billion year old fossils of blue green algae 
found in Australia. 

Scientists believe the early earth contained no oxygen gas, but did contain other gases, including nitrogen, 
carbon dioxide, carbon monoxide, water vapor, hydrogen sulfide and probably a few others. 

Geologists and other earth scientists use geologic time scales to describe when events occurred 
throughout the history of earth. 

The geological time scale of earth's past is organized according to events which took place during different 
periods on the time scale. 

Life on earth began about 3.5 to 4 billion years ago. 

The first life forms were single cell organisms, prokaryotic organisms, similar to bacteria. 

The first multicellular organisms did not appear until about 610 million years ago in the oceans. Some 
of the first multicellular forms included sponges, brown algae, and slime molds. 

Plants and fungi appeared roughly 500 million years ago. They were soon followed by arthropods (insects 
and spiders). 



169 



• Amphibians evolved about 300 million years ago, followed by mammals around 200 million years ago 
and birds around 100 million years ago. 

• Extinction of species is common; in fact, it is estimated that 99% of the species that have lived on the 
earth no longer exist. 

• Mass extinctions, such as the extinction of dinosaurs and many marine mammals, happened after major 
catastrophes such as volcanic eruptions and major earthquakes changed the environment. 

• There have been at least five major massive extinctions have occurred in the past 540 million years. 

• In each mass extinction, over 50% of animal species died. 

Review Questions 

1 . How do scientists determine the age of a rock or fossil today? 

2. How do we know the maximum possible age of the earth? 

3. How do we know the minimum possible age of the earth? 

4. How old is the earth, based on current evidence? 

5. Why is it difficult to determine how life started on earth? 

6. How long ago did life start on earth? 

7. When did mammals first appear on earth? 

8. What kinds of events are recorded on a geological time scale? 

Further Reading I Supplemental Links 

Stein, Sara, The Evolution Book, Workman, N.Y., 1986. 

Yeh, Jennifer, Modern Synthesis, (From Animal Sciences). 

Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859. 

Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003. 

Ridley, Matt, Genome, HarperCollins, 2000. 

Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006. 

Carroll, Sean B., The Making of the Fittest: DNAand the Ultimate Forensic Record of Evolution, Norton, 
2006. 

Dawkins, Richard, The Blind Watchmaker, W.W. Norton & Company, 1996. 

Dawkins, Richard, The Selfish Ge Oxford University Press, 1989. 

Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins, 
2006. 

Mayr, Ernst, What Evolution Is, Basic Books, 2001 . 

Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008. 



170 



• http://en.wikipedia.org/ 
Vocabulary 

Cambrian explosion A sudden burst of evolution that may have been triggered by some environmental 
changes that made the environment more suitable for a wider variety of life forms. 

extinct Something that does not exist anymore; a group of organisms that has died out 

without leaving any living representatives. 

mass extinction An extinction when many species go extinct during a relatively short period of time. 

radiometric dating A method to determine the age of rocks and fossils in each layer of rock; measures 
the decay rate of radioactive materials in each rock layer. 

stromolites Fossils made of algae and a kind of bacteria; some of the oldest fossils on Earth. 

Review Answers 

1 . Radiometric dating: a method of measuring the radioactivity of minerals in rocks. 

2. from the age of our solar system. 

3. from the age of the oldest rocks found on earth 

4. 4.04 to 4.47 billion years old 

5. Since we do not know exactly what the environment was like millions of years ago, we do not know what 
materials were available to start life. 

6. between 3-4 billion years ago 

7. about 200 million years ago 

8. Changes affecting the shape and conditions on earth, the development of new species, mass extinctions. 
Points to Consider 

The next chapter focuses on parkaryotic organisms. Remember, prokaryotes lived on this planet for two 
billion years before eukaryotic cells even existed. 

• Discuss with your class what you think are some of the characteristics, and some of the differences, of 
prokaryotic organisms. 



171 



172 



8. Prokaryotes 



Bacteria 

Lesson Objectives 

• Describe the cellular features of bacteria. 

• Explain the ways in which bacteria can obtain energy. 

• Explain how bacteria reproduce themselves. 

• Identify some ways in which bacteria can be helpful. 

• Identify some ways in which bacteria can be harmful. 

Check Your Understanding 

• How do prokaryotic and eukaryotic cells differ? 

Answer: Eukaryotic cells have a membrane-bound nucleus while prokaryotes do not. 

• What are some components of all cells, including bacteria? 
Answer: cell membrane, cytoplasm, etc. 
Introduction 

About 3.5 billion years ago, long before the first plants, people, or other animals appeared, prokaryotes 

were the first life forms on Earth. Recall that prokaryotes are single-celled organisms that lack a 

nucleus, and that the prokaryotes include bacteria and archaea. For at least a billion years, Bacteria 

and Archaea ruled the Earth as the only existing organisms. Even though life is much more diverse on 

Earth today, bacteria (singular: bacterium) are still the most abundant organisms on 

Earth. You probably know bacteria as "germs" that cause disease, but as you will see, they can also do 

many helpful things for the environment and humankind. 

Characteristics of Bacteria 

Bacteria are so small that they can only be visualized with a microscope. When viewed under the 

microscope, they have three distinct shapes. These three shapes allow bacteria to be 

classified by their shape. The bacilli are rod-shaped, the cocci are sphere-shaped, and the 

spirilli are spiral-shaped (Figures 1, 2 and 3). 



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Baccli 



Cocci 



Spirilli 



Figure 1 : Bacteria come in many different shapes. Some of the most common shapes are bacilli 

(rods), cocci (spheres), and spirilli (spirals). Bacteria can be identified and classified by their 

shape. 

(Source: http://commons.wikimedia.org/wiki/lmage: Bacteria morphologic forms simplified. svg, License: 
Public Domain) 




Figure 2: Escherichia coli is an example of bacteria that are rod-shaped, or bacilli. 

(Source: http://en.wikipedia.org/wiki/lmage: EscherichiaColi NIAID.jpg, License: Public Domain) 




174 



Figure 3: Staphylococcus aureus is an example of bacteria that are sphere-shaped, or cocci. 

(Source: http://commons.wikimedia.org/wiki/lmage: Staphylococcus_aureus_01 .jpg, License: Public Domain) 

Bacteria are surrounded by a cell wall consisting of peptidoglycan, a complex molecule consisting 

of sugars and amino acids. The cell wall is important for protecting the bacteria. In fact the cell wall 

is so important that some antibiotics, such as penicillin, work to kill bacteria by preventing the proper 

synthesis of the cell wall. In parasitic bacteria, which depend on a host organism for energy and 

nutrients, capsules or slime layers surround the cell wall help defend against the host's defenses. 

Recall that all prokaryotes, including the bacteria, lack the membrane-bound organelles and nucleus of 

eukaryotic cells (Figure 4). Like eukaryotic cells, however, prokaryotic cells do have 

cytoplasm, the fluid inside the cell; a plasma membrane, which acts as another barrier; and ribosomes, 

where proteins are assembled. The DNA of bacteria is mostly contained in a large circular strand, forming 

a single chromosome, that is compacted into a structure called the nucleoid. Many 

bacteria also have additional small rings of DNA known as plasmids. 



Capsule 
Cell wa 
Plasma membrane 



Cytoplasm 



Ribosomes 

Plasmid 
Pili 




Bacterial Flagellum 
Nucleoid (circular DNA) 



Figure 4: The structure of a bacterial cell is distinctive from the eukaryotic cell because of 

features such as an outer cell wall and the circular DNA of the nucleoid, and the lack 

of membrane-bound organelles. 

(Source: http://en.wikipedia.org/wiki/lmage: Average prokaryote cell-en. svg, License: Public Domain) 

Some bacteria also have tail-like structures called flagella (Figures 4 

and 5). The flagella assist the bacteria with movement. As the flagella rotate, they spin the 



175 



bacteria and propel them forward. 




Figure 5: The flagella facilitate movement in bacteria. Bacteria may have one, two, or many 

flagella - or none at all. 

(Source: http://en.wikipedia.org/wiki/lmage: EMpylori.jpg, License: Copyright 

Free Use) 

Obtaining Food and Energy 

Bacteria obtain energy and nutrients from a variety of different methods. Bacteria known as decomposers 

break down wastes and dead organisms into smaller molecules to obtain nutrients and energy. 

Photosynthetic bacteria use the energy of the sun, together with carbon dioxide, to make their 

own food (discussed in the Cell Functions chapter). An example of photosynthetic bacteria is 

cyanobacteria, as seen in Figure 6. 




Figure 6: Cyanobacteria are photosynthetic bacteria. These bacteria carry out all the reactions of photosyn- 
thesis within the cell membrane and in the cytoplasm; they do not need chloroplasts. 

(Source: http://en.wikipedia.org/wiki/lmage: Bluegreen_algae.jpg, License: Public Domain) 

Bacteria can also be chemotrophs. Chemotrophs obtain energy by breaking down 

chemical compounds in their environment, such as nitrogen-containing ammonia. This process is 



176 



important, for example, for the cycling of nitrogen through the environment. As nitrogen can 

not be made by living organisms, it must be continually recycled. Organisms need nitrogen to make 

organic compounds, such as DNA. 

Some bacteria depend on other organisms for survival. For example, mutualistic bacteria live in the root 

nodules of legumes, such as pea plants, and make nitrogen available to the plants; in this relationship 

both the bacteria and the plant benefit. Other bacteria are parasitic and can cause illness. In a 

bacterial parasitic relationship, the bacteria benefit and the other organism is harmed. Harmful 

bacteria will be discussed later in the lesson. 

Reproduction in Bacteria 

Bacteria reproduce asexually through binary fission. During binary fission the chromosome copies 

itself (replicates), forming two genetically identical copies, then the cell enlarges and 

divides into two new daughter cells. The two daughter cells are identical to the parent cell (Figure 7). 

Binary fission can happen very rapidly. Some species of bacteria have been shown to double their 

populations in less than ten minutes! (Figure 8) 

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Figure 7: Bacteria cells reproduce by binary fission, resulting in two daughter cells identical to 

the parent cell. 

(Source: http://commons.wikimedia.org/wiki/lmage: Bacilli_division_diagram.png, License: GNU_FD) 



177 




Figure 8: Bacteria can divide rapidly. This image is of a growing colony of E. coli bacteria. In the right envi- 
ronment the growth and division of two E. coli can form a colony of hundreds of bacteria in just a few hours. 

(Source: http://en.wikipedia.org/wiki/lmage: Growing_colony_of_E._coli.jpg, License: GNU-FD) 

Sexual reproduction does not occur in bacteria, but genetic recombination, the combining and 

exchange of DNA, does happen in bacteria through three different methods: conjugation, 

transformation, and transduction. In conjugation, DNA passes through the sex pilus, a 

hairlike extension on the surface of many bacteria, that temporarily joins two bacteria. 

In transformation, bacteria pick up pieces of DNA from their environment. In transduction, 

bacteriophages, viruses that infect bacteria, carry DNA from one bacteria to another. 

Helpful Bacteria 

Bacteria are crucial in nature since they are common decomposers, organisms that break down dead 

materials and waste products. This decomposition of dead organisms is necessary so that the nutrients in 

their bodies can be recycled back into the environment. This recycling of nutrients, such as 

nitrogen, is essential for living organisms; organisms cannot produce nutrients, so they must come 

from other sources. We get them from the food we eat; plants get them from 

the soil. How do these nutrients get into the soil? One way is from the actions of 

decomposers. So without decomposers, we would eventually run out of materials essential for our 

survival. We also depend on bacteria to decompose our wastes in sewage treatment plants. 

Bacteria also help you digest your food. Several species of bacteria, such as E. coli, are 

found in large amounts in your digestive tract. In fact, bacteria cells outnumber your own cells in your 



178 



gut! 

Bacteria are involved in producing some foods. Yogurt is made by using bacteria to ferment milk, and 
cheese can also be made from milk with the help of bacteria (Figure 9). Furthermore, fermenting 
cabbage with bacteria produces sauerkraut. 




Figure 9: Yogurt is made from milk fermented with bacteria. The bacteria ingest natural milk 

sugars and release lactic acid as a waste product, which causes proteins in the milk to form 

into a solid mass, which becomes the yogurt. 

(Source: http://www.flickr.com/photos/momthebarbarian/2441500/, License: CC-Attribution) 

In the laboratory, bacteria can be altered to provide us with a variety of useful materials. 

Bacteria can be used as tiny factories to produce desired chemicals and medicines. For example, insulin, 

which is necessary to treat people with diabetes, can be produced from bacteria. Through the process 

of transformation, the human gene for insulin is placed into bacteria. The bacteria then turn 

that gene into a protein. The protein can be isolated and used to treat patients. The mass 

production of insulin by bacteria made this medicine more affordable for patients. 

Harmful Bacteria 

There are also ways that bacteria can be harmful to humans and other animals. Various species of bacteria 

are responsible for many types of human illness, including strep throat, tuberculosis, pneumonia, 

leprosy, and Lyme disease. The Black Death (also known as Plague), which killed at least one third of 

Europe's population in the 1300's, is believed to have been caused by the bacterium Yersinia pestis. 

Bacterial contamination can also lead to outbreaks of food poisoning. Raw eggs and undercooked meats 
can 



179 



contain bacteria that can cause digestive tract problems. Foodborne infection can be prevented by cooking 
meat 

thoroughly and washing surfaces that have been in contact with raw meat. Washing of hands before and 

after handling food is also important. 

Some bacteria also have the potential to be used as biological weapons by terrorists. An example is 

anthrax, a disease caused by the bacterium Bacillus anthracis. Since inhaling the spores of this 

bacterium can lead to a fatal infection, it is a dangerous weapon. In 2001 , an act of terrorism in the 

United States involved B. anthracis spores sent in letters through the mail. 

Lesson Summary 

• Bacteria contain a cell wall containing peptidoglycan and a single chromosome contained in the nucleoid. 

• Bacteria can obtain energy through several means including photosynthesis, decomposition, and para- 
sitism. 

symbiosis and chemosynthesis. 

• Bacteria reproduce through binary fission. 

• Bacteria are important decomposers in the environment and aid in digestion. 

• Some bacteria can be harmful when they contribute to disease, food poisoning, or biological warfare. 

Review Questions 

1 . What are prokaryotes? (Intermediate) 

2. What are the possible shapes that bacteria can have? (Challenging) 

3. What is the purpose of the flagella? (Beginning) 

4. Describe the DNA of bacteria. (Challenging) 

5. How do bacteria reproduce? (Intermediate) 

6. How do bacteria assure genetic recombination? (Challenging) 

7. What is a chemoautotroph? (Beginning) 

8. How do cyanobacteria obtain energy? (Beginning) 

9. How are bacteria important in nature? (Intermediate) 

10. How can you avoid becoming sick from the bacteria that cause food poisoning? (Intermediate) 
Further Reading I Supplemental Links 
http://www.bt.cdc.gov/agent/anthrax 
http://www.cdc.gov/ncidod/dvbid/plague/index.htm 
http://www.cdc.gov/nczved/dfbmd/disease_listing/salmonellosis_gi.html 



180 



http://www.ucmp.berkeley.edu/bacteria/bacteria.html 

http://commtechlab.msu.edu/sites/dlc-me/zoo 

http://www.cellsalive.com/cells/bactcell.htm 

http://www.microbeworld.org/microbes/bacteria 

http://en.wikipedia.org/wiki 

Vocabulary 

bacilli Rod-shaped bacteria or archaea. 

binary fission Type of asexual reproduction where a parent cell divides into two identical daughter cells. 

cocci Sphere-shaped bacteria or archaea. 

chemotrophs Organisms that obtain energy by oxidizing compounds in their environment. 

conjugation The transfer of genetic material between two bacteria. 

cyanobacteria Photosynthetic bacteria. 

decomposers Organisms that break down wastes and dead organisms and recycle their nutrients back 
into the environment. 

flagella Long, tail-like appendages that allow movement. 

nucleoid The prokaryotic DNA consisting of a condensed single chromosome. 

peptidoglycan Complex molecule consisting of sugars and amino acids that makes up the bacterial cell 
wall. 

plasmid Ring of accessory DNA in bacteria. 

prokaryotes Organisms that lack a nucleus and membrane-bound organelles; bacteria and archaea. 

transduction Transfer of DNA between two bacteria with the aid of a bacteriaphage. 

transformation Changing phenotypes due to the incorporation ("taking up") of foreign DNA from the envi- 
ronment. 

spirilli Spiral-shaped bacteria or archaea. 

Review Answers 

1 . These are single-celled organisms that lack a nucleus or membrane-bound organelles. 

2. The bacilli are rod-shaped, the cocci are sphere-shaped, and the spirilli are spiral-shaped. 

3. The purpose of the flagella is movement. 

4. Bacteria have a single circular chromosome and accessory rings of DNA called plasmids. 

5. Bacteria reproduce through the process called binary fission. 

6. Bacteria assure genetic recombination through conjugation, a process where DNA is passed between 
cells; transformation, where pieces of DNA are picked up from the environment; transduction, where 
bacteriophages transfer DNA. 

7. Organisms that can synthesize organic molecules and obtain energy by oxidizing inorganic compounds. 

8. Cyanobacteria obtain energy through photosynthesis. 

9. They are decomposers that recycle nutrients from dead organisms and wastes. 



181 



10. Avoid eating undercooked meats and raw eggs; wash surfaces in contact with raw meat. 

Points to Consider 

• In the next section we will discuss the Archae. "Archae" shares the same root word as "archives" and 
"archaic," so what do you think it means? 

• What do you think the earliest life forms on Earth looked like? 

• How do you think these early life forms obtained energy? 

Archaea 

Lesson Objectives 

• Identify the differences between archaea and bacteria. 

• Explain how the archaea can obtain energy. 

• Explain how the archaea reproduce. 

• Discuss the unique habitats of the archaea. 

Check Your Understanding 

• What are the three shapes of bacteria? 

Answer: The bacilli are rod-shaped, the cocci are sphere-shaped, and the spirilli are spiral-shaped. 

• How do bacteria reproduce? 

Answer: Through binary fission, producing genetically identical organisms. 

• How can bacteria be harmful? 

Answer: Bacteria can cause diseases such as strep throat. They can also be involved with food 

poisoning and biological warfare. 

Introduction 

For many years, archaea were classified as bacteria. However, when modern techniques allowed scientists 
to compare the DNA of the two prokaryotes, they found that there were two distinct types of prokaryotes, 
which they named archaea and bacteria. Even though the two groups might seem similar, archaea have 
many features that distinguish them from bacteria. 

1 . The cell walls of archaea are distinct from those of bacteria. In most archaea the cell wall is assembled 
from surface-layer proteins, providing both chemical and physical protection. The cell wall acts as a barrier, 
preventing macromoleculesfrom coming into contact with the cell membrane. In contrast to bacteria, most 
archaea lack peptidoglycan in their cell walls. 

2. The plasma membranes of the archaea also are made up of lipids that are distinct from those in other 
organisms. 

3. Furthermore, the ribosomal proteins of the archaea resemble those of eukaryotic cells; the ribisimal proteins 
of archaea are different from those found in bacteria. 

Although archaea and bacteria share some fundamental differences, they are still similar in many ways. 
182 



1 . They both are unicellular, microscopic organisms that can come in a variety of shapes (Figure 1 ). 

2. Both archaea and bacteria have a single circular chromosome of DNA and lack membrane-bound or- 
ganelles. 

3. Like bacteria, the archaea can have flagella to assist with movement. 




Figure 1: Archaea shapes can vary widely, but some are bacilli, or rod-shaped. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Arkea.jpg, License: GNU_FD) 

Obtaining Food and Energy 

Most archaea are chemotrophs and derive their energy and nutrients from breaking down molecules from 
their environment. A few species of archaea are photosynthetic and capture the energy of sunlight; 
chemotrophs do not capture the energy from sunlight. Unlike bacteria, which can be parasites and are known 
to cause a variety of diseases, there are no known archaea that act as parasites. Some archaea do live 
within other organisms, however, but form mutualistic relationships with their host, where both the archaea 
and host benefit. In other words, they actually assist the host in some way, for example by helping to digest 
food. 

Reproduction 

Like bacteria, reproduction in archaea is asexual. Archaea can reproduce through binary fission, where a 

parent cell divides into two genetically identical daughter cells. Archaea can also reproduce asexually 

through budding and fragmentation, where pieces of the cell break off and form a new cell, also producing 

genetically identical organisms. 

Types of Archaea 

The first archaea described were unique in that they could survive in extremely harsh environments where 

no other organisms could survive. For example, the halophiles, which means "salt-loving," live in 

environments with high levels of salt (Figure 2). They have been identified in the Great Salt Lake 

in Utah and in the Dead Sea between Israel and Jordan, which have salt concentrations several times that 

of the oceans. 



183 




Figure 2: Halophiles, like the Halobacterium shown here, require high salt concentrations. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Hal0bacteria.jpg, License: Public Domain) 

The thermophiles live in extremely hot environments (Figure 3). For example, they can grow in 

hot springs, geysers, and near volcanoes. Unlike other organisms, they can thrive in temperatures near 

100°C, the boiling point of water! 




Figure 3: Thermophiles can thrive in hot springs and geysers, such as this one, the Excelsior Geyser in the 
Midway Geyser Basin of Yellowstone National Park, Wyoming. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Excelsi0r_geyser.jpg, License: Public Domain) 



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Methanogens can also live in some strange places, such as swamps, and inside the guts of cows and 

termites. They help these animals break down cellulose, a tough carbohydrate made by plants 

(Figure 4). This would be an example of a mutualistic relationship. Methanogens are named for 

their waste product, methane, which they make as they use hydrogen gas to reduce carbon dioxide and 
gain energy. Methane is a greenhouse gas and therefore contributes to global warming (see the Environ- 
mental Problems chapter). Therefore, the rate of methane released in swamps is of interest to scientists 

studying climate change. 




Figure 4: Cows are able to digest grass with the help of the methanogens in their gut. 
(Source: http://www.flickr.com/photos/gertcha/858758122/, License: CC-Attribution) 
Although archaea are known for living in unusual environments, like the Dead Sea, inside hot 
springs, and in the guts of cows, they also live in more common environments. For example, new 
research shows that archaea are abundant in the soil and among the plankton in the ocean. Therefore, 
scientists are just beginning to discover some of the important roles that archaea have in the 
environment. 

Lesson Summary 

• Archaea are prokaryotes that differ from bacteria somewhat in their DNA and biochemistry. 

• Most archaea are chemotrophs but some are photosynthetic or form mutualistic relationships. 

• Archaea reproduce asexually through binary fission, fragmentation, or budding. 

• Archaea are known for living in extreme environments. 

Review Questions 

1 . What domains include the prokaryotes? (Beginning) 

2. How are the cell walls of archaea different from those of bacteria? (Intermediate) 



185 



3. How do archaea obtain energy? (Challenging) 

4. How do archaea reproduce? (Intermediate) 

5. Where do halophiles live? (Intermediate) 

6. Where do thermophiles live? (Intermediate) 

7. How did methanogens get their name? (Intermediate) 

8. Name an example of a mutualistic relationship with archaea. (Challenging) 
Further Reading I Supplemental Links 
http://www.ucmp.berkeley.edu/archaea/archaea.html 
http://www.microbeworld.org/microbes/archaea 
http://www.ncbi.nlm.nih.gov/pubmed/21 12744?dopt=Abstract 
http://www.popsci.com/environment/article/2008-07/they-came-underseas 
http://www.sciencedaily.com/releases/2006/06/0606051 91 500.htm 
http://en.wikipedia.org/wiki/Archaea 

Vocabulary 

archaea Single-celled, prokaryotic organisms that are distinct from bacteria. 

halophiles Organisms that live and thrive in very salty environments. 

methanogens Organisms that live in swamps or in the guts of cows and termites and release methane 
gas. 

thermophiles Organisms that live in very hot environments, such as near volcanoes and in geysers. 

Review Answers 

1 . Bacteria and Archaea domains include the prokaryotes. 

2. Unlike the bacteria, archaea do not contain peptidoglycan in their cell walls. 

3. Most are chemotrophs, some are photosynthetic, and some are mutalistic. 

4. Asexually through budding, binary fission, or fragmentation. 

5. They live in extremely salty environments. 

6. They live in extremely hot environments, such as near volcanoes and in geysers. 

7. They produce methane gas as a waste produce. 

8. An example is in the guts of termites and cows, archaea help to break down cellulose. 

Points to Consider 

• In the next chapter we will move on to the protists and fungi. How do you think they are different from 
archaea and bacteria? 



186 



Can you think of some ways that fungi can be helpful? 
Can you think of some ways that fungi can be harmful? 



187 



188 



9. Protists and Fungi 



Protists 

Lesson Objectives 

• Explain why protists cannot be classified as plants, animals, or fungi. 

• List the similarities that exist between most protists. 

• Identify the three subdivisions of the organisms in the kingdom Protista. 

Check Your Understanding 

• What are some basic differences between a eukaryotic cell and a prokaryotic cell? 

• List some characteristics that all cells have. 

Introduction 

So what's a protist? Is it an animal or plant? Protists are organisms that belong to the kingdom Protista. 
These organisms, all eukaryotes and mostly unicellular, do not fit neatly into any of the other kingdoms. 
You can think about protists as all eukaryotic organisms that are neither animals, nor plants, nor fungi. Even 
among themselves, they have very little in common - very simple structural organization and a lack of 
specialized structures are all that unify them as a group. Although the term protista was coined by Ernst 
Haeckel in 1866, the kingdom Protista was not an accepted classification in the scientific world until the 
1960s. 

What are Protists? 

These unique and varied organisms demonstrate such unbelievable differences that they are sometimes 
called the "junk drawer kingdom". This kingdom contains the eukaryotes that cannot be classified into any 
other kingdom. Most protists, such as the ones shown in Figure 1, are so tiny that they can be seen only 
with a microscope. Protists are mostly unicellular eukaryotes that exist as independent cells. However, a 
few protists are multicellular and surprisingly large. The protists that do form colonies (are multicellular) do 
not, however, show cellular specialization or differentiation into tissues. Cellular specialization is a major 
feature of multicellular organisms absent in these protists. For example, kelp is a multicellular protist and is 
over 100-meters long. 

A few characteristics unify the protists: 

1 . they are eukaryotic which means they have a nucleus 

2. most have mitochondria 

3. many are parasites 

4. they all prefer aquatic or at least moist environments. 

For classification, the protists are divided into three groups: animal-like protists, plant-like protists, and fungi- 
like protists. But remember they are not animals or plants or fungi, they are protists (Figure 2). As there are 
many different types of protists, the classification of protists can be difficult. Recently, molecular analysis has 
been used to confirm evolutionary relationships among protists. These molecular studies compare DNA 
sequences. Protists with higher amounts of common DNA sequences are evolutionary closer related to 
each other. Protists are widely used in industry and in medicine. 



189 




Figure 1 : Protists come in many different shapes. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Protist_collage.jpg, License: GNU Free Documentation) 




Figure 2: This slime mold is a protist. Slime molds had previously been classified as fungi but are now 
placed in the kingdom Protista. Slime molds live on decaying plant life and in the soil. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Slime_mold.jpg, License: GNU Free Documentation) 

Protists Obtain Food 

Protists need to perform the necessary cellular functions to stay alive. These include the need to grow and 
reproduce, the need to maintain homeostasis, and the need for energy. So they need to obtain food to provide 
the energy to enable these functions. 

So how are animal-like, plant-like, and fungi-like protists distinguished from each other? Mainly through how 
they get their carbon. Of course, carbon is essential in the formation of organic compounds: carbohydrates, 
lipids, proteins, and nucleic acids. You get it from eating, as do other animals. 



190 



For such simple organisms, protists get their food in a complicated process. Although there are many pho- 
tosynthetic protists (such as the algae discussed in the Plant-like Protists section below) that get their energy 
from sunlight, many others still must swallow their food through a process like endocytosis. Endocytosis 
was discussed in the Cell Functions chapter. 

When a protist is ready to eat, it will wrap its cell wall and cell membrane around its prey, which is usually 
bacteria. In doing so, it creates a food vacuole or a sort of "food storage compartment." Next the protist pro- 
duces toxins which paralyze its prospective dinner. Once paralyzed, the food material simply moves by force 
of gravity through the vacuole and into the cytoplasm of the hungry protist. Other protists are parasitic, and 
absorb nutrients meant for their host, harming the host in the process. 

Animal-like Protists 

Animal-like protists are called protozoa. Protozoa are unicellular eukaryotes that share certain traits with 
organisms in the animal kingdom. Those traits are mobility and heterotrophy. Animal-like protists are het- 
erotrophs which mean they get their carbon from outside sources — in other words, they eat organic mate- 
rials. Animal-like protists are very tiny measuring only about 0.01-0. 5mm. Animal like protists include the 
zooflagellates, ciliates, and the sporozoans (Figure 3). 




Figure 3: Euglena are animal-like protists. Over 1000 species of Euglena exist and are used in industry in 
the treatment of sewage. 

(Source: http://en.wikipedia.org/wiki/Euglena, License: Public Domain) 

Although most protists obtain nutrition through pinocytosis, some protists literally "eat with their tails". The 
tail of a protist is a flagellum and these protists are called flagellates. Flagellates acquire oxygen and nitrogen 
by constantly whipping the flagellum back and forth in a process of filter-feeding. The whipping of the flagellum 
creates a current that brings food into the protist. 

A flagellum (plural: flagella), is a tail-like structure that projects from the cell body of certain prokaryotic 
and eukaryotic cells, and it usually functions in helping the cell move. A flagellum is a cellular structure and 
not an organelle. Prokaryotic cells may also have flagella. 

Different Kinds of Animal-like Protists 

Are there different types of animal-like protists? How are they distinguished? You can distinguish one from 
the other based on how they get around or rather, by their method of locomotion. For example, flagellates 
have long flagella or tails. Flagella rotate in a propeller-like fashion. An example of a flagellate is the Try- 
panosoma, which causes African sleeping sickness. Other protists have what is called a "transient pseu- 
dopodia" or a moving fake foot. Here's how it works. The cell surface extends out a membrane and the force 
of this membrane propels the cell forward. An example of a protist with a pseudopod is the amoeba. Another 
way to move if you are a protist is by the movement of cilia. The Paramecium has cilia that propel it. Cilia 
are thin, tail-like projections that extend about 5-10 micrometers outwards from the cell body. Cilia beat 
back and forth, propelling the protist along. A few protists are non-mobile such as the toxoplasma. Protists 
such as the toxoplasma form spores and are known as sporozoans; these protists but do not have any 



191 



mobility themselves. 
Plant-like Protists 

Plant-like protists are autotrophs. This means that they produce complex organic compounds from simple 
inorganic molecules using a source of energy such as sunlight. Plant-like protists live in soil, in seawater, 
on the outer covering of plants, in ponds and lakes. Protists like these can be unicellular, or multicellular. 
Some protists, such as kelp live in huge colonies in the ocean. Plant-like protists are essential to the envi- 
ronment; they produce oxygen (a product of photosynthesis) which sustains other organisms and they play 
an essential role in aquatic food chains. Plant-like protists are classified into a number of basic groups (see 
Table 1). 

Table 1: Plant-like Protists 



Phylum 


Description 


Number (ap- 
proximate) 


Example 


Chlorophyta 


green algae - re- 
lated to higher 
plants 


7,500 


Chlamydomnas, 
Ulva, Volvox 


Rhodophyta 


red algae 


5,000 


Porphyra 


Phaeophyta 


brown algae 


1,500 


Macrocystis 


Chrysophyta 


diatoms, golden- 
brown algae, 
yellow-green al- 
gae 


12,000 


Cyclotella 


Pyrrophyta 


dinoflagellates 


4,000 


Gonyaulax 


Euglenophyta 


euglenoids 


1,000 


Euglena 




Figure 4: Red algae are a very large group of protists making up about 5,000-6,000 species. They are 
mostly multicellular, live in the ocean. Many red algae are seaweeds and help create coral reefs. 

(Source: http://en.wikipedia.org/wiki/Red_alga, License: GNU Free Documentation) 

Fungus-like Protists 

Fungus-like protists are heterotrophs that have cell walls and reproduce by forming spores. Fungus-like 
protists mostly immobile but some develop movement at some point in their lives. There are essentially 
three types of fungus-like protists: water molds, downy mildews, and slime molds (see Table 2). Slime molds 
represent the characteristics of the fungus-like protists. Most slime mould measure about one or two cen- 
timeters, but a few slime molds are as big as several meters. They are often bright colors such as a vibrant 
yellow. Others are brown or white. Stemonitis is a kind of slime mould which forms small brown bunches 
on the outside of rotting logs. Physarum polycephalum lives inside rotting logs and is a gooey mesh of yellow 



192 



"threads" that are a several centimeters long. Fuligo, sometimes called "vomit mold", is a yellow slime mold 
found in decaying wood. 

Table 2: Fungus-like Protists 



Protist 


Source of Car- 
bon 


Environment 


Characteristics 


omycetes: water 
molds 


decompose re- 
mains, parasites 
of plants and 
animals 


most live in wa- 
ter 


Causes a range 
of diseases in 
plants; common 
problem in 
greenhouses 
where the organ- 
ism kills newly 
emerged 
seedlings; have 
been employed 
as biocontrol 
agents; includes 
the downy 
mildews, which 
are easily identi- 
fiable by the ap- 
pearance of 
white "mildew" 
on leaf surfaces. 


Mycetozoa: 
slime molds 


dispose of dead 
plant material, 
feed on bacteria 


common in soil, 
on lawns, and in 
the forest com- 
monly on decidu- 
ous logs 


Includes the cel- 
lular slime mold, 
which involves 
numerous indi- 
vidual cells at- 
tached to each 
other, forming 
one large "super- 
cell," essentially 
a bag of cyto- 
plasm contain- 
ing thousands of 
individual nuclei. 
The plasmodial 
slime molds 
spend most of 
their lives as indi- 
vidual unicellular 
protists, but 
when a chemical 
signal is se- 
creted, they as- 
semble into a 
cluster that acts 
as one organ- 
ism. 



193 




Figure 5: An example of a slime mold. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Slime_Mold_Olympic_National_Park_North_Fork_Sol_Duc.jpg, 
License: Public Domain) 




Figure 6: An aquatic insect nymph attacked by water mold. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Water_mold.jpg, License: GNU Free Documentation) 

Importance of Protists 

Earth would be uninhabitable if it were not for the 80 different groups of organisms called protists. Protists 
produce almost one-half of the oxygen on the planet, decompose and recycle nutrients that humans need 
to live, and make up a huge portion of the food chain. Many protists are commonly used in medical research. 
For example, medicines made from protists are used in treatment of high blood pressure, digestion problems, 
ulcers, and arthritis. Other protists are used in molecular biology and genetics studies. Slime molds are used 
to analyze the chemical signals used in directing cellular activities. Protists are also valuable in industry. 
Carrageenan, extracted from red algae, is used as a gel to solidify puddings, ice cream, and candy. Chem- 
icals from other kinds of algae are used in the production of many kinds of plastics. 

Lesson Summary 

• Protists are highly diverse organisms that belong to the kingdom Protista. 



194 



• Protists are divided into three subgroups: animal-like protists, plant-like protists, fungus-like protists. 

• Animal-like protists are unicellular eukaryotes that that share certain traits with organisms in the animal 
kingdom such as mobility and heterotrophy. 

• Plant-like protists are unicellular or multicellular autotrophs that live in soil, in seawater, on the outer 
covering of plants, in ponds and lakes. 

• Fungus-like protists, such as water molds, downy mildews, and slime molds, are heterotrophs that repro- 
duce by forming spores. 

Review Questions 

1 . List the unifying characteristics of protists? (Beginning) 

2. List two ways that protists obtain food. (Beginning) 

3. Describe the characteristics of an animal-like protist. (Intermediate) 

4. Describe the characteristics of a plant-like protist. (Intermediate) 

5. Describe the characteristics of a fungi-like protist. (Intermediate) 

6. Name three kinds of fungi-like protists. (Beginning) 

7. Write a convincing essay demonstrating the importance of protists to life on earth. (Intermediate) 

8. Imagine that you are a scientist delivering a paper called "Protists: the Junk-Drawer Kingdom" What would 
you say in your paper to explain your choice of title? (Challenging) 

Further Reading I Supplemental Links 

King, Katie and Ball, Jacqueline, Protists and Fungi. 2003 Gareth Stevens Publishing. 

Marguilis, L., Corliss, J.O., Melkonian, M.,and Chapman, D.J. (Editors) 1990. Handbook of Protoctista. Jones 
and Bartlett, Boston. 

Jahn,T.L., Bovee, E.C. & Jahn, F.F. 1979 How to Know the Protozoa. 2nd ed. Wm. C. Brown Publishers, 
Div. of McGraw Hill, Dubuque, Iowa. 

Patterson, D.J. 1996. Free-Living Freshwater Protozoa: A Colour Guide. John Wiley & Sons, NY. 

Streble H., Krauter D. 1988. Life in a waterdrop. Microscopic freshwater flora and fauna. An identification 
book. 

http://www.ucmp.berkeley.edu/alllife/eukaryotasy.html 

http://www.funsci.com/fun3_en/protists/entrance.htm 

http://www.biology.arizona.edu/cell_bio/tutorials/pev/main.html 

http://en.wikipedia.org/wiki 

Vocabulary 

autotroph Organism that produces complex organic compounds from simple inorganic 

molecules using a source of energy such as sunlight. 

cilia Thin, tail-like projections that extend about 5-1 micrometers outwards from the 

cell body; beat back and forth, propelling the protist along. 



195 



filter-feeding Characteristic of flagellates; acquire oxygen and nitrogen by constantly whipping 

the flagellum back and forth; creates a current that brings food into the protist. 

flagellum A tail-like structure that projects from the cell body of certain prokaryotic and eu- 

karyotic cells, and it usually functions in helping the cell move. 

heterotroph Organism which obtains carbon from outside sources. 

protist Eukaryotic organism that belongs to the kingdom Protista; not a plant, animal or 

fungi. 

protozoa animal-like protists 

transient pseudopodia A moving fake foot; the cell surface extends out a membrane and the force of 

this membrane propels the cell forward. 

Review Answers 

1 . Protists are eukaryotic, which means they have a nucleus, most have mitochondria, many are parasites, 
and they all prefer aquatic or at least moist environments. 

2. Filter feeding and pinocytosis. 

3. Animal-like protists are mobile and heterotrophs. Animal-like protists are very tiny measuring only about 
0.01-0. 5mm. 

4. Plant-like protists are autotrophs. This means that they produce complex organic compounds from simple 
inorganic molecules using a source of energy such sunlight. Plant-like protists live in soil, in seawater, 
on the outer covering of plants, in ponds and lakes. Protists like these can be unicellular, or multicellular. 

5. A fungi-like protist is a heterotroph that has cell walls and reproduces by forming spores. 

6. Water molds, downy mildews, and slime molds. 

7. Earth would be uninhabitable if it were not for protists. Protists produce almost one-half of the oxygen on 
the planet, decompose and recycle nutrients that humans need to live, and make up a huge portion of 
the food chain. Many protists are commonly used in medical research. Protists are also valuable in industry. 
Chemicals from other kinds of algae are used in the production of many kinds of plastics. 

8. Protists are organisms that belong to the kingdom Protista. These organisms, all eukaryotes and mostly 
unicellular, do not fit neatly into any of the other kingdoms. Even among themselves, they have very little 
in common-very simple structural organization and a lack of specialized structures are all that unify them 
as a group. 

Points to Consider 

• Fungi comprise one of the eukaryotic kingdoms. Think about what might distinguish a fungi-like protist 
from a true fungus? 

• Given the vast differences between the protists discussed in this lesson, think about the possibilities of 
dividing this kingdom into additional kingdoms. How might that division be accomplished? Is that a good 
idea or would it just lead to confusion? 



Fungi 

Lesson Objectives 

• Describe the characteristics of fungi. 

• Identify structures that distinguish fungi from plants and animals. 



196 



• Explain how fungi can be used in industry. 

Check Your Understanding 

• What is a significant difference between a protist and other eukaryotic organisms? 

• What are some of the distinguishing characteristics of fungus-like protists? 

Introduction 

Ever notice blue-green mold growing on a loaf of bread? Do you like your pizza with mushrooms? Has a 
physician ever prescribed an antibiotic for you? If so, then you have encountered fungi. Fungi are organisms 
that belong to the kingdom Fungi. Our ecosystem needs fungi. Fungi help decompose matter and make 
nutritious food for other organisms. Fungi are all around us and are useful in many ways to the natural world 
and to humans in industry. 



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Figure 1: These many different kinds of organisms demonstrate the huge diversity within the kingdom Fungi. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Fungi_c0llage.jpg, License: CC-BY-SA2.5) 

What are Fungi? 

If you had to guess, would you say fungus is a plant or animal? Scientists used to debate about which 
kingdom to place fungi in. Finally they decided that fungi were plants. But they were wrong. Now scientists 
know that fungi are not plants at all. Fungi are very different from plants. Fungi belong to their own kingdom 
called the kingdom Fungi. 

Plants are autotrophs, meaning that they make their own "food" using the energy from sunlight. Fungi are 
heterotrophs, which means that they obtain their "food" from outside of themselves. In other words, they 
must "eat" or ingest their food like animals or many bacteria do. 

Yeasts, molds, and mushrooms are all different kinds of fungi. There may be as many as 1 .5 million species 
of fungi. You can easily see bread mold and mushrooms without a microscope, but most fungi you cannot 
see. Fungi is either too small to be seen without a microscope or it lives where you cannot see it easily such 
as deep in the soil, or under decaying logs, or inside plants or animals. Some fungi even live in or on top of 
other fungi. 



197 




Figure 2: The blue in this blue cheese is actually mold. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Blue_Stilton_Quarter_Fror1t.jpg) 

Fungi and Symbiotic Relationships 

If it were not for fungi, many plants would go hungry. In the soil fungi grow closely around the roots of plants. 
Then as they form that close relationship, the plant and the fungus "feed" one another. The plant provides 
glucose and sucrose to the fungus that the plant makes through photosynthesis which the fungus cannot 
do. The fungi then provides minerals and water to the roots of the plant. This form of helping each other out 
is called mycorrhizal symbiosis. Mycorrhizal means "roots" and symbiosis means "relationship" between 
organisms. 




Figure 3: This mushroom and tree live in symbiosis with each other. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Oudemansiella_nocturnum.jpg) 

Lichens 

Have you ever seen an organism called a lichen? Lichens are crusty, hard growths that you might find on 
trees, logs, walls, and rocks. Although lichens may not be the prettiest organisms in nature, they are unique. 
A lichen is really two organisms that live very closely together — a fungus and a bacteria or algae. The cells 
from the algae or bacteria live inside the fungi. Each organism provides nutrients for the other. Consequently, 
a lichen is the result of the symbiosis between a fungus and an another organism. 

The earliest scientist to study lichens was Beatrix Potter. You might have heard of her as the author and il- 
lustrator of the Peter Rabbit stories. Before Beatrix Potter became a famous author, she was a botanist and 
studied hundreds of different kinds of fungi. She was the first person to explain the symbiotic relationship 
between bacteria and fungi. She even presented a scientific paper to the British scientific community in 
1897. 



198 



Fungi and Insects 

Many insects have a symbiotic relationship with certain types of fungi. For example, ants and termites grow 
fungi in underground "fungus gardens" that they create. Then when the ants or termites have eaten a big 
meal of wood or leaves, they eat some fungus from their gardens. The fungus helps them digest the cellulose 
in the wood or leaves. The two species are actually dependent on each other for survival. Ambrosia beetles 
live in the bark of trees. Like ants and termites, they grow fungi inside the bark of trees where they live and 
use it to help digest their food. 

Fungi as Parasites 

Although lots of symbiotic relationships help both organisms, sometimes one of the organisms is harmed. 
When that happens, the organism that benefits and is not harmed is called a parasite. Have you ever heard 
of Dutch elm disease? In the late 1960's elm trees in the United States began to die. Since then much of 
the species has been eliminated. The disease was caused by a fungus that acted as a parasite. The fungus 
that killed the trees was carried by beetles that inoculated the tree with the fungus. The tree tried to stop the 
growth of the fungus by blocking its own ability to gain water. However, without water the tree soon dies. 

Some parasitic fungi cause human diseases such as athlete's foot and ringworm. These fungi feed on the 
outer layer of warm, moist skin. 

Fungi as Predators 

It might seem that fungi growing on a tree trunk or a mushroom in your yard are passive and participating 
in very little activity, but did you know that some fungi are actually hunters? Some fungi trap nematodes. A 
nematode is a kind of a worm and is often dinner to fungi. These hungry fungi live deep in the soil where 
they set traps for unsuspecting nematodes by making a circle with their hyphae. Hyphae are sort of the 
"arms and legs" of a fungus; they look like cobwebs and can be sticky. Fungi set out circular rings of hyphae 
with a lure inside which brings the nematode inside the fungus. 




Figure 5: Hyphae are the cobwebby arms and legs of fungi. 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/6/66/Penicillium.jpg) 

Fungi are Good Eaters 

Fungi can grow fast because they are such good eaters. Fungi have lots of surface area and this large 
surface area "eats". Surface area is how much exposed area an organism has compared to their overall 
volume, and in the mushroom for example, most of that surface area is actually underground. They also 
have special enzymes that they can squirt into their environment which helps them digest large organic 
molecules. Sort of like how you might cut up your meat or vegetables before eating, fungi "cut up" large 



199 



molecules such as sugars, proteins, and lipids into smaller molecules. Then the fungi absorb the nutrients 
into their cells. 

Fungi Body Parts 

Fungi have a cell wall, hyphae, and specialized structures for reproduction. The hyphae are thread-like 
structures which interconnect and bunch up into mycelium. Ever see mold on a damp wall or on old bread? 
The thing that you are seeing is really mycelia. The hyphae and mycelium help the fungi absorb nutrients 
from living hosts. Other specialized structures are used in sexual reproduction. One example is a fruiting 
body. A mushroom is a fruiting body, which is the part of the fungus that produces the spores. Spores are 
the basic reproductive units of fungi. 

Fungi Reproduction 

Reproduction of fungi is different for different fungi. Many fungi reproduce both sexually or asexually, while 
some reproduce only sexually and some only asexually. Asexual reproduction takes only one parent and 
sexual reproduction takes two parents. 

Asexual Reproduction 

Fungi reproduce asexually through three methods: spores, budding, and mycelial fragmentation. Asexual 
spores are formed by the fungi and released to create new fungi. Have you ever seen a puffball? A puffball 
is a kind of fungus that has thousands of spores in a giant ball. Eventually the puffball bursts and releases 
the spores in a huge "puff'. In budding, the fungus grows part of its body which eventually breaks off. The 
broken-off piece becomes a "new" organism. Many fungi can reproduce by mycelial fragmentation or splitting 
off of the mycelia. A fragmented piece of mycelia can eventually produce a new colony of fungi. 

Asexual reproduction is faster and produces more fungi than sexual reproduction. For some species of fungi, 
asexual reproduction is the only way possible to reproduce. Asexual reproduction is controlled by many 
different factors, including environmental conditions such as the amount of sunlight and C0 2 the fungus re- 
ceives, as well as the availability of food. 

Sexual Reproduction 

Almost all fungi can reproduce with meiosis. Meiosis is a type of cell division where haploid cells are produced 
(discussed in chapter titled Cell Division, Reproduction and DNA). But meiosis in fungi is really different 
from sexual reproduction in plants or animals. 

Meiosis occurs in diploid cells and is a process that produces haploid cells. A diploid cell is a cell with two 
sets of chromosomes-one from each parent. A haploid cell has one set of chromosomes. In meiosis, the 
chromosomes duplicate once, and then after two more divisions, four haploid cells are produced. Each 
haploid cell has half the chromosome numberof the parent cell. However, in fungi, meiosis occurs right after 
two haploid cells fuse, producing four haploid cells. Mitosis then produces a haploid multicellular "adult" or- 
ganism or haploid unicellular organisms. Mitosis is cell division that results in two genetically identical offspring 
cells. 

Other Sexual Processes 

Some species of fungi exchange genetic material by parasexual processes. This means that some haploid 
nuclei in the fungi cells may fuse and form diploid nuclei. These nuclei rarely exist and are usually very un- 
stable. Chromosomes are lost during later mitotic divisions which sometimes makes the offspring fungus 
genetically different from the parents. 

Classification of Fungi 

Scientists used to think that fungi were members of the Plant kingdom. They thought this because fungi had 
several similarities to plants. For example, fungi and plants are usually sessile with a leaf or flower that is 



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attached to a stem. Also: 

• Both fungi and plants have similar morphology or structure. 

• Plants and fungi live in the same kinds of habitats, such as growing in soil. 

• Plants and fungi both possess a cell wall; animals cells do not have a cell wall. 

But scientists now know that fungi are their own separate kingdom — the kingdom Fungi. And that they 
separated nearly one billion years ago. 

Physiological and Morphological Traits 

There are a number of characteristics that distinguish fungi from other eukaryotic organisms. 

1 . Fungi cannot make their own food like plants can since they do not have any of the right equipment for 
photosynthesis. Fungi are more like animals and some bacteria in that they have to obtain their food from 
outside sources. 

2. The cell walls in lots of species of fungi is chitin. Chitin is a nitrogen-containing material that you find in 
the shells of animals such as beetles and lobsters. But the cell wall of a plant is not made of chitin but 
rather a carbohydrate called cellulose. 

3. Unlike many plants, most fungi do not have a good vascular system. A vascular system is the way that 
an organism transports fluids such as water and nutrients. In all plant the vascular system is made up of 
structures called xylem and phloem. But fungi do not have xylem or phloem. This lack of vascular structures 
distinguishes fungi from plants. 

4. However, one characteristic is entirely unique to fungi and does not exist at all in animals or plants. That 
characteristic is hyphae which combine in groups called mycelium, as described above. 

The Evolution of Fungi 

Fungi appeared during the Paleozoic Era, a geologic time period lasting from about 570 million to 248 million 
years ago, and the time when fish, insects, amphibians, reptiles, and land plants appeared. The first fungi 
were most likely aquatic, and had flagellum that released spores. The first land fungi probably appeared in 
the Silurian period (443 million years ago to about 416 million years ago), a geologic period during which 
land plants also appeared. 

Roles of Fungi 

Fungi are found all over the globe in many different kinds of habitats. Fungi even thrive in deserts. Most 
fungi however are found on land rather than in the ocean, but some species live only in marine habitats. 
Fungi are extremely important to these ecosystems because they are one of the major decomposers of organic 
material in most terrestrial ecosystems. Scientists have estimated that there are nearly 1.5 million species 
of fungi. 

Importance of Fungi for Human Use 

Humans use fungi for food preparation or preservation and other purposes. For example, yeasts are required 
for fermentation of beer, wine and bread. Some fungi are used in the production of soy sauce and tempeh, 
a stable source of protein, like tofu, found in South East Asia. Mushrooms are used in the diet of people all 
over the globe. Otherfungi are producers of antibiotics, such as penicillin. The chitin in the cell walls of fungi, 
have wound healing properties. 



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Figure 6: Baker's yeast or Saccharomyces cerevisiae, a single-cell fungus, is used in the baking of bread 
and in making wine and beer through fermentation. 

(Source: http://en.wikipedia.0rg/wiki/lmage:S_cerevisiae_under_DIC_micr0sc0py.jpg, License: GNU Free 
Documentation) 

Edible and Poisonous Fungi 

Some of the best known types of fungi are mushrooms-both edible and poisonous. Many species are grown 
commercially, but others must be harvested from the wild. When you order a pizza with mushrooms or add 
them to your salad, you are most likely eating Agaricus bisporus, the most commonly eaten species. Other 
mushroom species are gathered from the wild for people to eat or for commercial sale. 

Have you ever eaten blue cheese? Do you know what makes it blue? You guessed it. Fungus. For certain 
types of cheeses, producers inoculate milk curds with fungal spores to promote the growth of mold which 
makes the cheese blue. Molds used in cheese production are safe for humans to eat. 

Many mushroom species are poisonous to humans — some mushrooms will simply give you a stomach ache 
while others may kill you. Some mushrooms you can eat when they are cooked but are poisonous when 
raw. 




Figure 7: Some of the best known types of fungi are the edible and the poisonous mushrooms. 
(Source: http://en.wikipedia.0rg/wiki/lmage:Asian_mushrooms.jpg, License: CC-BY-SA 2.0) 



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Fungi in the Biological Control of Pests 

Some fungi work as natural pesticides. For example in agriculture, some fungi may be used to limit or kill 
harmful organisms like mites, pest insects, certain weeds, worms, and other fungi that harm or even kill 
crops. 

Lesson Summary 

Fungi are in their own kingdom based on their structures, ways of obtaining food, and on their means of 
reproduction. 

Fungi live with other organisms in symbiotic relationships. 

Fungi reproduce asexually, sexually and parasexually. 

Fungi appeared during the Paleozoic Era. 

Fungi are widely used in industry and medicine. 

Review Questions 

1 . What two characteristics distinguishes fungi from plants? (Beginning) 

2. How many species of fungi exist? (Beginning) 

3. Define mycorrhizal symbiosis. (Intermediate) 

4. Describe the symbiotic relationship of a lichen. (Intermediate) 

5. How was Beatrix Potter important to the scientific world? (Intermediate) 

6. Describe the relationship between the ambrosia beetle and fungi? (Intermediate) 

7. Name two human diseases caused by fungus. (Beginning) 

8. When you see mold what body part of the fungus are you observing? (Challenging) 

9. Describe asexual reproduction in fungi. (Challenging) 
1Q Describe sexual reproduction in fungi. (Challenging) 

Further Reading I Supplemental Links 

Money, Nicholas, The Triumph of Fungi: A Rotten History. Oxford University Press, 2006. 

Webster, Robert and Weber, Roland, Introduction to Fungi. Cambridge University Press, 2007. 

Moore-Landecker, Elizabeth, Fundamentals of Fungi. Benjamin Cummings, 1996. 

http://www.tolweb.org/Fungi 

http://www.ucmp.berkeley.edu/fungi/fungi.html 

http://www.perspective.com/nature/fungi 



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http://en.wikipedia.Org/wiki/lmage:DecayingPeachSmall.gif 
Vocabulary 



asexual reproduction 

budding 

chitin 

fruiting body 

heterotroph 
hyphae 

lichen 

meiosis 

mycelial fragmentation 

mycelium 
mycorrhizal symbiosis 

parasite 
spores 



Reproduction involving only one parent; fungi reproduce asexually through three 
methods: spores, budding, and mycelial fragmentation. 

Asexual reproduction in which the fungus grows part of its body which eventually 
breaks off; the broken-off piece becomes a new organism. 

A nitrogen-containing material found in the cell wall of fungi; also found in the 
shells of animals such as beetles and lobsters. 

Specialized structure used in sexual reproduction; part of the fungus that produces 
the spores. 

Organism which obtains carbon ("food") from outside of themselves. 

Thread-like structures which interconnect and bunch up into mycelium; helps 
bring food, such as a worm, inside the fungus; : the "arms and legs" of a fungus. 

A symbiotic relationship between a fungus and a bacteria or algae; each organism 
provides nutrients for the other. 

A type of cell division where haploid (one set of chromosomes) cells are produced. 

Asexual reproduction involving splitting off of the mycelia; a fragmented piece of 
mycelia can eventually produce a new colony of fungi. 

Help the fungi absorb nutrients from living hosts; composed of hyphae. 

A relationship between fungi and the roots of plants where both benefit; the plant 
provides glucose and sucrose to the fungus that the plant makes through photo- 
synthesis; the fungi provides minerals and water to the roots of the plant. 

The organism that benefits in a relationship between two organisms in which one 
is harmed. 

The basic reproductive units of fungi. 



Review Answers 

1 . Eukaryotes and heterotrophs 

2. 1.5 million 

3. In the soil fungi grow closely around the roots of plants. Then as they form a close relationship, the plant 
and the fungus "feed" one another. The plant provides glucose and sucrose to the fungus that the plant 
makes through photosynthesis which the fungus cannot do. The fungi then provides minerals and water 
to the roots of the plant. 

4. A lichen is really two organisms that live very closely together — a fungus and a bacteria or algae. The 
cells from the algae or bacteria live inside the fungi. Each organism provides nutrients for the other. 

5. She was the first person to explain the symbiotic relationship between bacteria and fungi. She even pre- 
sented a scientific paper to the British scientific community in 1897. 

6. Ambrosia beetles live in the bark of trees. Like ants and termites, they grow fungi inside the bark of trees 
where they live and use it to help digest their food. 

7. Athlete's foot and ringworm 

8. The hyphae or mycelia 

9. Fungi reproduce asexually through three methods: spores, budding, and mycelial fragmentation. Asexual 
spores are formed by the fungi and released to create new fungi. In budding, the fungus grows part of 
its body which eventually breaks off. The broken-off piece becomes a "new" organism. Many fungi can 
reproduce by mycelial fragmentation or splitting off of the mycelia. A fragmented piece of mycelia can 



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eventually produce a new colony of fungi. 

1Q Almost all fungi can reproduce with meiosis. Meiosis is a type of cell division where eggs and sperm are 
produced. But meiosis in fungi is really different from sexual reproduction in plants or animals. In fungi, 
meiosis occurs right after two haploid cells fuse, and mitosis then produces a haploid multicellular "adult" 
organism or haploid unicellular organisms. Mitosis is cell division that results in two identical offspring 
cells. 

Points to Consider 

• Plants are fascinating organisms and are widely diverse. Although scientists used to think that fungi were 
plants, we now know that plants are fungi are separate. In this lesson we have discussed fungi. Now 
think about what sets plants apart from fungi? 



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10. Plants 



Introduction to Plants 

Lesson Objectives 

• Describe the major characteristics that distinguish the Plant Kingdom. 

• Describe plants' major adaptations for life on land. 

• Explain plants' reproductive cycle. 

• Explain how plants are classified. 

Check Your Understanding 

• What are the major differences between a plant cell and an animal cell? 

• What is photosynthesis? 

Introduction 

Plants have adapted to a variety of environments, from the desert to the tropical rain forest to our lakes and 
oceans. In each environment, plants have become crucial to supporting animal life. First, plants provide 
animals with food. In a forest, for example, caterpillars munch on leaves while birds eat berries and deer 
eat grass. Furthermore, plants make the atmosphere friendly for animals. Plants absorb animals' "waste" 
gas, carbon dioxide, and release the oxygen all animals need for cellular respiration. Finally, plants provide 
cover and shelter for animals. A bird can take refuge from predators in a shrub and use twigs to make a 
nest high in a tree (Figure 1). Without plants, animals would not be able to survive. 




Figure 1 : These bird eggs are benefiting from the cover of a plant; plant materials make up the nest, and 
when the eggs hatch, the young birds will eat plant products like seeds and berries. 



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(Source: http://www.flickr.com/photos/j-pocztarski/2495538130/, License: CC-Attribution) 
What Are Plants? 

From tiny mosses to extremely large trees (Figure 2), the organisms classified into the Plant Kingdom have 
three main distinguishable features. 

They are all: 

• eukaryotic 

• photosynthetic 

• multicellular 

Recall that eukaryotic organisms also include animals, protists, and fungi; eukaryotic cells have true nuclei 
that contain DNAand membrane-bound organelles such as mitochondria. As discussed in the Cell Functions 
chapter, photosynthesis is the process by which plants capture the energy of sunlight and use carbon 
dioxide from the air to make their own food. Lastly, plants must be multicellular. Recall that some protists, 
like diatoms, are eukaryotic and photosynthetic; however, diatoms are not considered plants. Diatoms are 
a major group of algae, and are mostly unicellular. 




Figure 2: There is great diversity in the plant kingdom, from tiny mosses to huge trees. 



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(Source: http://www.flickr.com/photos/gnaharro/2170397378/, License: CC-Attribution) 
Adaptations For Life On Land 

Much evidence suggests that plants evolved from freshwater green algae (Figure 3). For example, green 
algae and plants both have the carbohydrate cellulose in their cell walls and they share many of the same 
pigments. (For a review of plant cells, see the Cells and Their Structures chapter.) So what separates green 
algae, which are protists, from green plants? 




Figure 3: The ancestor of plants is green algae. This picture shows a close up of algae on the beach. 

(Source: http://www.flickr.eom/photos/73109955@N00/172202630/, License: CC-Attribution) 

One of the main features that distinguishes plants from algae is the retention of the embryo during develop- 
ment. In plants, the embryo develops and is nourished in the female reproductive structure after fertilization. 
Algae do not retain the embryo. This was the first feature to evolve that separated the plants from the green 
algae. Plant reproduction will be discussed in the following section. 

Although the retention of the embryo is the only adaptation shared by all plants, over time other adaptations 
for living on land also evolved. In early plants, a waxy layer called a cuticle evolved to help seal water in 
the plant and prevent water loss. However, the cuticle also prevents gases from entering and leaving the 
plant easily. Recall that the exchange of gasses - taking in carbon dioxide and releasing oxygen - occurs 
during the process of photosynthesis. Therefore, along with the cuticle, small pores in the leaves called 
stomata also evolved (Figure 4). The stomata can open and close depending on weather conditions; when 
it's hot and dry the stomata can stay closed to conserve water. The stomata can open again to permit gas 
exchange when the weather cools down. 



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Figure 4: Stomata are pores in leaves that allow gasses to pass through, but they can be closed to conserve 
water. 

(Source: http://commons.wikimedia.org/wiki/lmage: Arabidopsis-epiderm-stomata2.jpg, License: GNU-FD) 

A later adaption for life on land was the evolution of vascular tissue. Vascular tissue is specialized tissue 
that transports water, nutrients, and food in plants. In algae, vascular tissue is not necessary since the entire 
body is in contact with the water. But on land, water may only be present deep in the ground. Vascular tissue 
delivers water and nutrients from the ground up and food down into the rest of the plant. The two vascular 
tissues are xylem and phloem. Xylem is responsible for the transport of water and mineral nutrients from 
the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. 
Phloem mainly carries the sugars made during photosynthesis to the parts of the plant where it is needed. 

Plant Reproduction and Life Cycle 

Alteration of generations describes the lifecycleof a plant (Figure 5). In alternation of generations, the plant 
alternates between a sporophyte that has two sets of chromosomes (diploid) and a gametophyte that has 
one set of chromosomes (haploid). Briefly, alternation of generations can be summarized in the following 
four steps: follow along in Figure 5 as you read through the steps. 

1. The gametophyte produces the gametes, sperm and egg, by mitosis. Remember, gametes are haploid. 

2. Then the sperm fertilizes the egg, producing a diploid zygote that develops into the sporophyte. 

3. The sporophyte produces haploid spores by meiosis. 

4. The haploid spores undergo mitosis, developing into the gametophyte. 

As we will see in the following lessons, the generation in which the plant spends most of its lifecycle differs 
between various plants. In the plants that first evolved, the gametophyte takes up the majority of the lifecycle 
of the plant. During the course of evolution, the sporophyte became the major stage of the lifecycle of the 
plant. In flowering plants, the female gametophyte is retained within the sporophyte and the male gametophyte 
is the pollen. 



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Figure 5: In ferns, the sporophyte is dominant and produces spores that germinate into a gametophyte; 
after fertilization the sporophyte is produced. Ferns will be discussed in further detail in the next lesson. 

(Source: http://commons.wikimedia.org/wiki/lmage: Altemation_of_generations_in_ferns.png, License: GNU- 
FD) 

Classification of Plants 

The Plant Kingdom is formally divided into 12 phyla, and these phyla are subdivided into four groups: 

1. nonvascular plants 

2. seedless vascular plants 

3. nonflowering plants 

4. flowering plants 

Figure 6 portrays some of the rich diversity of this kingdom. These four groups are based on the evolutionary 
history of significant features in plants. The first significant feature to evolve in the Plant Kingdom, after the 
retention of the embryo, was vascular tissue. Vascular tissue allowed the transport of water and food 
throughout the plant. The phyla that were around before the evolution of the vascular tissue are known as 
the nonvascular plants (without vascular tissue to move water, nutrients and food). The next significant 
step in the evolutionary history of plants was the development of the seed. Plants that evolved vascular tissue 
but do not have seeds are the seedless vascular plants. The final major evolutionary event in the Plant 



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Kingdom was the evolution of flowers and fruits. Plants with vascular tissue and seeds but without flowers 
are the gymnosperms. The plants that have all these features and also fruits and flowers are the an- 
giosperms. These four groups are the focus of the next two lessons. 




Figure 6: The plant kingdom contains a diversity of organisms. Note that Volvox in the upper left is a protist, 
not a plant. 

(Source: http://en.wikipedia.org/wiki/lmage: Diversity_of_plants_image_version_3.png, License: GNU-FD) 

Lesson Summary 

• Plants are multicellular photosynthetic eukaryotes that evolved from green algae. 

• Plants have several adaptive features for living on land, including a cuticle, stomata, and vascular tissue. 

• Plants are informally divided into four groups: the nonvascular plants, the seedless vascular plants, the 
nonflowering plants (gymnosperms) and the flowering plants (angiosperms). 

Review Questions 

1 . How are plants necessary for animal life? (Intermediate) 

2. Compare and contrast a typical plant to a photosynthetic protist like a diatom. (Challenging) 

3. Plants evolved from green algae. How are they different from green algae? (Intermediate) 

4. What strategies have plants evolved for life on land? (Intermediate) 



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5. What is the purpose of the stomata? (Challenging) 

6. What term describes the plant life cycle? (Beginning) 

7. What is the diploid stage of the alteration of generations? (Beginning) 

8. Which generation of the alternation of generations is dominant in early plants? (Beginning) 

9. What is the term for plants that lack vascular tissue? (Beginning) 

1Q What is the term for plants that have flowers and bear fruit? (Beginning) 

Further Reading I Supplemental Links 

http://www.ucmp.berkeley.edu/plants/plantae.html 

http://www.bioedonline.org/slides/slide01. cfm?q=%22Plantae%22 

http://www.wisc-online.com/objects/index_tj. asp?objlD=BIO804 

http://www.perspective.com/nature/plantae 

http://www.wisc-online.com/objects/index_tj. asp?objlD=BIO804 

http://www.perspective.com/nature/plantae 

http://plants.usda.gov 

http://en.wikipedia.org/wiki 

Vocabulary 

alteration of generations The plant lifecycle, which alternates between a haploid gametophyte and a 

diploid sporophyte. 

angiosperms Plants that flower and bear fruit. 

cuticle Waxy layer that aids water retention in plants. 

gamete haploid sex cell; egg or sperm 

gametophyte Haploid generation of the alteration of generations life cycle; produces gametes. 

gymnosperms Seed plant where seeds are not enclosed by a fruit. 

nonvascular plants Plants that do not have vascular tissue to conduct food and water. 

sporophyte Diploid generation of the alteration of generations; produces haploid spores. 

stomata Small pores on the underside of leaves that can regulate the passage of gasses 

and moisture. 

vascular tissue Tissues that conduct food, water, and nutrients in plants. 

Review Answers 

1. They provide food, shelter, oxygen, etc. 

2. Diatoms and plants are both eukaryotic and photosynthetic. Diatoms are unicellular, while plants are 
multicellular. 

3. Plants retain the embryo. 

4. cuticle, stomata, vascular tissue 

5. To regulate gas exchange and conserve moisture. 



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6. alteration of generations 

7. sporophyte 

8. gametophyte 

9. nonvascular plants 
1Q angiosperm 

Points to Consider 

• Can you think of examples of plants that do not have seeds? 

• If a plant does not have seeds, how can it reproduce? 

Seedless Plants 

Lesson Objectives 

• Name examples of nonvascular seedless plants. 

• Name examples of vascular seedless plants. 

• Explain the reproduction strategies of seedless plants. 

• Describe the ways seedless plants impact humankind. 

Check Your Understanding 

' What is a plant? 

• How are plants classified? 

Introduction 

What might you think a forest would have looked like millions of years ago? Or tens of millions of years ago? 
Or hundreds of millions of years ago? Probably very different than today. In this lesson the focus will be on 
the very first and most ancient plants: the nonvascular seedless plants and the vascular seedless plants. 
These plants have had a great impact on all our lives. Over 300 million years ago, during the Carboniferous 
period, forests looked very different than they do today. Seedless plants grew as tall as today's trees in vast 
swampy forests (Figure 1 ). The remains of these forests formed the fossil fuel coal that we depend on today. 
Although most of these giant seedless plants are now extinct, smaller relatives still remain. 



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StcitikolilfiiforiiiiiMiMi [f. 



:— " 




Figure 1 : Seedless plants were dominant during the Carboniferous period, as illustrated by this drawing. 

(Source: http://en.wikipedia.org/wiki/lmage: Meyers_b15_s0272b.jpg, License: Public Domain) 

Nonvascular Seedless Plants 

Since the nonvascular seedless plants lack vascular tissue, they also do not have true roots, stems, or 
leaves. Remember that vascular tissue moves water, food and nutrients throughout the plant. By definition, 
roots, stems and leaves must contain vascular tissue. However, nonvascular plants do often have a "leafy" 
appearance and can have stem-like and root-like structures. These plants must also remain very short in 
stature due to their lack of ability to conduct nutrients and water up a stem. The appearances of the nonva- 
scular plants vary, however, and they are classified into three phyla: the mosses, the hornworts, and the 
liverworts. 

The mosses, phylum Bryophyta, are most often recognized as the green "fuzz" on damp rocks and trees 
in a forest. If you look closely, you will see that most mosses have tiny stem-like and leaf-like structures. 
This is the gametophyte stage. Remember from lesson 1 that the gametophyte is haploid. The gametophyte 
produces the gametes that, after fertilization, develop into the diploid sporophyte. The sporophyte forms a 
distinctive capsule, called the sporangium, which releases spores (Figure 2). 



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Figure 2: Sporophytes sprout up on stalks from this bed of moss gametophytes. Notice that both the 
sporophytes and gametophytes exist at the same time, 

(Source: http://www.flickr.com/photos/enygmatic/39281344/, License: CC-SA) 

The hornworts, phylum Anthocerophyta, get their name from their distinctive hornlike sporophytes, and 
"wort" which comes from the Anglo-Saxon word for herb. The hornlike sporophytes grow from a base of 
flattened lobes, which are the gametophytes (Figure 3). They tend to grow in moist and humid areas. 




Figure 3: In hornworts, the "horns", the sporophytes are rise up from the leaflike gametophyte. 

(Source: http://en.wikipedia.Org/wiki/lmage:Anthoceros_levis.jpg, License: GNU_FD) 

Liverworts, phylum Hepatophyta, have two distinct appearances- they can either be leafy like mosses or 
flattened and ribbon-like. Liverworts get their name from the type with the flattened bodies which can resemble 
a liver (Figure 4). Liverworts can often be found along stream beds. 



216 




Figure 4: Liverworts with a flattened, ribbon-like body are called thallose liverworts. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Liverw0rt.jpg, License: GNU_FD) 

Vascular Seedless Plants 

As their name implies, vascular seedless plants have vascular tissue but do not have seeds. Vascular tissue 
is specialized tissues which conduct water and nutrients throughout the plant. Vascular tissue allowed these 
plants to grow much taller than nonvascular plants, forming the ancient swamp forests mentioned previously. 
Most of these large vascular seedless plants are now extinct, but their smaller relatives still remain. Seedless 
vascular plants include the club mosses, the ferns, the horsetails, and the whisk ferns. 

Club mosses, in the phylum Lycophyta, are so named because they can look similar to mosses (Figure 
5). Club mosses are not true mosses, though, because they have vascular tissue. The "club" part of the 
name comes from club-like clusters of sporangia in some types of club mosses. The resurrection plant is 
also a club moss. It shrivels and turns brown when it dries out, but then quickly recovers and turns green 
when watered again. 








Figure 5: Club mosses can superficially resemble mosses, but they have vascular tissue. 

(Source: http://www.flickr.com/photos/benimoto/2447130740/, License: CC-Attribution) 

Ferns, in the phylum Pterophyta, are the most common seedless vascular plants (Figure 6). They typically 
have large divided leaves called fronds. In most ferns, fronds develop from a curled-up formation called a 
fiddlehead (Figure 7). The fiddlehead resembles the curled decoration on the end of a stringed instrument, 



217 



such as a fiddle. Leaves unroll as the fiddleheads grow and expand. Ferns grow in a variety of habitats, 
ranging in size from tiny aquatic species to giant tropical plants. 




Figure 6: Ferns are common in the understory of the tropical rainforest. 

(Source: http://www.flickr.com/photos/kats_pics/186991752/, License: CC-Attribution) 



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Figure 7: The first leaves of most ferns appear curled up into fiddleheads. 

(Source: http://www.flickr.com/photos/victoriapeckham/493717420/, License: CC-Attribution) 

The horsetails, the phylum Sphenophyta, have hollow, ribbed stems and are often found in marshes (Figure 
8). Whorls of tiny leaves around the stem make the plant look like a horse's tail, but these soon fall off and 
leave the photosynthetic hollow stem. The stems are rigid and rough to the touch as they are coated with 
abrasive silicates. Because of their scratchy texture, these plants were once used as scouring pads for 
cleaning dishes. 




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Figure 8: Horsetails are common in marshes. 

(Source: http://www.flickr.com/photos/backpackphotography/267389290/, License: CC-Attribution) 

The whisk ferns, the phylum Psilophyta, have green branching stems with no leaves, so they resemble a 
whisk broom (Figure 9). Another striking feature of the whisk ferns is their spherical yellow sporangia. 




Figure 9: Whisk ferns have no leaves and bear yellow sporangia. 

(Source: http://commons.wikimedia.org/wiki/lmage: Psilotum.jpg, License: GNUFD) 

Reproduction of Seedless Plants 

Seedless plants can reproduce asexually or sexually. Some seedless plants, like hornworts and liverworts, 
can reproduce asexually through fragmentation. When a small fragment of the plant is broken off, it can 
form a new plant. 

Like all plants, nonvascular plants have an alternation of generations lifecycle. In the lifecycle of the nonva- 
scular seedless plants, the gametophyte is dominant. The gametophyte is photosynthetic and normally de- 
scribed as the plant. The male gametophyte produces flagellated sperm that must swim to the egg formed 
by the female gametophyte. For this reason, sexual reproduction must happen in the presence of water; 
hence the nonvascular plants tend to live in moist environments. Following fertilization, the sporophyte forms. 
The sporophyte is connected to and dependent on the gametophyte. The purpose of the sporophyte is to 
produce spores that will develop into gametophytes and start the cycle over again. 

For the seedless vascular plants, the sporophyte tends to be dominant. For example, in ferns the gametophyte 
is a tiny heart-shaped structure, and the leafy plant we recognize as a fern is the sporophyte (as shown in 
lesson 1, Figure 5). The sporangia of ferns are often on the underside of the fronds (Figure 10). Like the 
nonvascular plants, ferns also have flagellated sperm that must swim to the egg. But unlike the nonvascular 
plants, once fertilization takes place, the gametophyte will die and the sporophyte will thrive independently. 



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Figure 10: This fern is producing spores underneath its fronds. 

(Source: http://www.flickr.com/photos/theequinest/2514581801/, License: CC-Attribution) 

Why Seedless Plants Are Important 

The greatest influence of seedless plants on human society was in the formation of the fossil fuel coal millions 
of years ago. Coal is burned to provide energy. But some seedless plants still have uses in society today. 
Sphagnum, also called peat moss, is commonly used by gardeners to improve soils since it has a great 
ability to absorb and hold water (Figure 11). Ferns are also a familiar fixture in many gardens. Besides being 
prized for their ornamental value, the fiddleheads of certain species of ferns are used in gourmet food. Some 
species of ferns, like the maidenhair fern, are believed by some people to have medicinal qualities. 




Figure 11: Sphagnum, or peat moss, is commonly added to soil to aid water retention. 

(Source: http://commons.wikimedia.org/wiki/lmage: Schultz_Sphagnum_Peat_Moss.jpg, License: CC-SA) 

Lesson Summary 

• Nonvascular seedless plants include mosses, liverworts, and hornworts. 

• Vascular seedless plants include club mosses, ferns, whisk ferns, and horsetails. 

• Nonvascular seedless plants tend to have a dominant gametophyte while vascular seedless plants tend 
to have a dominant sporophyte. 



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• Mosses and ferns are used commonly in gardening. 

Review Questions 

1. What is vascular tissue? (Beginning) 

2. What is an example of a nonvascular seedless plant? (Beginning) 

3. What is an example of a vascular seedless plant? (Beginning) 

4. Compare and contrast the fern gametophyte and sporophyte. (Challenging) 

5. Compare and contrast the whisk fern (Psilophyta) and the ferns (Pterophyta). (Challenging) 

6. Compare and contrast mosses and club mosses. (Challenging) 

7. What are some uses of the seedless plants to gardeners? (Beginning) 

8. What are some of the distinguishing features of horsetails? (Intermediate) 

9. What does the sporophyte of the hornwort look like? (Beginning) 
1Q Explain reproduction by fragmentation. (Intermediate) 

Further Reading I Supplemental Links 

http://www.cavehill.uwi.edu/FPAS/bcs/bl14apl/bryo1.htm 

http://www.microscopy-uk.org.uk/mag/indexmag.html 

http://www.microscopy-uk.org.uk/mag/artjul98/jpmoss.html 

http://www.biologycorner.com/bio2/notes_plants.html 

http://forestencyclopedia.eom/p/p1893 

http://www.hiddenforest.co.nz/plants/clubmosses/clubmosses.htm 

http://amerfernsoc.org/; http://www.washjeff.edu/greenhouse/Pnudum 

http://en.wikipedia.org/wiki 

Vocabulary 

club mosses Seedless vascular plants that resemble mosses. 

ferns Seedless vascular plants that have large, divided fronds. 

hornworts Seedless nonvascular plants with hornlike sporophytes. 

horsetails Seedless vascular plants with hollow, rigid stems. 

liverworts Seedless nonvascular plants that can have flattened bodies resembling a liver. 

mosses Seedless nonvascular plants with tiny stem-like and stem-like structures. 

whisk ferns Seedless vascular plants that have branching stems and yellow globular sporangium. 

Review Answers 

1 . conductive tissue for food, water, and nutrients 

2. liverwort, hornwort, moss 



222 



3. fern, club moss, whisk fern, horsetail 

4. The fern gametophyte is small and heart shaped, while the sporophyte has fronds. 

5. The fern has fronds while the whisk fern has no leaves. They are both vascular seedless plants. 

6. Both are seedless plants, but only the club moss has vascular tissue. The gametophyte is dominant in 
mosses and the sporophyte is dominant in club mosses. 

7. Peat moss is a soil amendment; ferns are common garden plants. 

8. hollow, rigid stems rough with silica 

9. hornlike 

1Q Asexual reproduction, by which a small fragment of the plant breaks off and forms a new plant. 

Points to Consider 

• Can you think of examples of plants that have seeds? 

• Can you think of a plant that has seeds but no flowers or fruits? 

• Why do you think having flowers is beneficial to a plant? 

Seed Plants 

Lesson Objectives 

• Describe the importance of the seed. 

• Explain the ways in which seeds are dispersed. 

• Define and give examples of Gymnosperms. 

• Define and give examples of Angiosperms. 

• Explain some uses of seed plants. 

Check Your Understanding 

• What are the two types of seedless plants? 

• How do seedless plants reproduce? 

Introduction 

If you've ever seen a plant grow from a tiny seed, then you might realize that seeds are rather amazing 
structures. The seed allows a plant embryo to survive droughts, harsh winters, and other conditions that 
would kill an adult plant. The tiny plant embryo can simply stay dormant, in a resting state, and wait for the 
perfect conditions for growth before it sprouts. In fact, some seeds can stay dormant for hundreds of years! 
Another impressive feature of the seed is that it provides stored food for the seedling after it sprouts. This 
greatly increases the chances that the tiny plant will survive. So being able to produce a seed is a very 
beneficial adaptation, and as a result, seed plants have been very successful. Although the seedless plants 
were here on Earth first, today there are many more seed plants than seedless plants. Recall that there are 
two different groups of seed plants: the Gymnosperms, which do not have flowers or fruits, and Angiosperms, 



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which do have flowers and fruits. 
Seeds and Seed Dispersal 

For a seed plant species to be successful, the seeds must be dispersed, or scattered out in various directions. 
If the seed are distributed in a variety of areas, there is a better chance that some of the seeds will find 
suitable conditions for growth. Furthermore, for plants to establish themselves in new areas, such as areas 
formed after a glacier retreat, the seeds must somehow reach that new site. To aid with seed dispersal, 
some plants have evolved special features to encourage their seeds to move long distances. 

One such strategy is to allow the wind to carry the seeds. With special adaptations in the seeds, the seeds 
can be carried long distances by the wind. For example, you might have noticed how the "fluff' of a dandelion 
moves swiftly in the breeze. Each piece of fluff carries a seed to a new location. Or if you look under the 
scales of pine cone, you would see tiny seeds with "wings" that allow these seeds to be carried away by the 
wind. Maple trees also have specialized fruits with wing-like extensions that aid in seed dispersal, as shown 
in Figure 1. 




Figure 1: Maple trees have fruits with "wings" that help the wind disperse the seeds. 

(Source: http://www.flickr.com/photos/imajilon/2681470706/, License: CC-Attribution) 

Another common seed dispersal strategy that some flowering plants utilize is to produce a fleshy fruit around 
the seeds. Animals that eat the seeds aid in the dispersal of the seeds inside. Berries, citrus fruits, cherries, 
apples, and a variety of other types of fruits are all adapted to be attractive to animals (Figure 2). Some 
seeds can pass through an animal's digestive tract unharmed and germinate after they are passed out with 
the feces. 



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Figure 2: Fleshy fruits aid in seed dispersal since animals eat the fruits and carry the seeds to a new location. 

(Source: http://www.flickr.com/photos/heydrienne/22080973/, License: CC-Attribution) 

Some non-fleshy fruits are especially adapted for animals to carry them on their fur. You might have returned 
from a walk in the woods to find burrs stuck to your socks. These burs are actually specialized fruits that 
carry seeds to a new location. 

Gymnosperms 

Plants with "naked" seeds, meaning they are not enclosed by a fruit, are called Gymnosperms. Instead, 
the seeds of Gymnosperms are usually found in cones. There are four phyla of gymnosperms: 

1 . Coniferophyta, common name conifers 

2. Cycadophyta, common name cycads 

3. Ginkgophyta, Ginkgo trees 

4. Gnetophyta, common name gnetophytes 

The Conifers, members of the phylum Coniferophyta, are probably the gymnosperms that are most familiar 
to you. The conifers include pines, firs, spruces, cedars, and the coastal redwoods in California that are 
tallest living vascular plants. The name of this group signifies that the plants bear their reproductive structures 
in cones, but this is not a characteristic unique to this phylum (Figure 3). Conifer pollen cones are usually 
very small, while the seed cones are larger. Pollen contains gametophytes that produce the male gamete 
of seed plants. The pollen, which is a fine to coarse powder-like material, is carried by the wind to fertilize 
the seed cones (Figure 4). 



1 


{r r J% — 







Figure 3: A red pine, which bears seeds in cones, is an example of a conifer. 



225 



(Source: http://www.flickr.com/photos/odalaigh/2456681765/, License: CC-Attribution) 




Figure 4: The end of a pine tree branch bears the male cones that produce the pollen. 

(Source: http://www.flickr.com/photos/foxypar4/2036547013/, License: CC-Attribution) 

The Conifers are important to humankind since they have many uses. They are important sources of lumber 
and are also used to make paper. Resins, the sticky substance you might see oozing out of a wound on a 
pine tree, are collected from conifers to make a variety of products, such as the solvent turpentine and the 
rosin used by musicians and baseball players. The sticky rosin improves the pitcher's hold on the ball or 
increases the friction between the bow and the strings to help create music from a violin or other stringed 
instrument. 

The Cycads, in the phylum Cycadophyta, are also Gymnosperms. They have large, finely-divided leaves 
and grow as short shrubs and trees in tropical regions. Like the conifers, they produce cones, but the seed 
cones and pollen cones are always on separate plants (Figure 5). One type of cycad, the sago palm, is a 
popular landscape plant. During the Age of the Dinosaurs (about 65 to 200 million years ago) the cycads 
were the dominant plants. So you can imagine dinosaurs grazing on cycad seeds and roaming through cycad 
forests. 




Figure 5: Cycads bear their pollen and seeds in cones on separate plants. 

(Source: http:/w/ww.flickr.com/photos/glennf/20342176/, License: CC-Attribution) 

Ginkgo trees, in the phylum Ginkgophyta, are unique because they are the only species left in the phylum, 
although there are many other species in the fossil record that have gone extinct (Figure 6). Therefore, the 
Ginkgo tree is sometimes considered a "living fossil". The Ginkgo tree survived as it was widely cultivated 
in China, especially around Buddhist temples. The Ginkgo tree is also a popular landscape tree today in 
American cities because it is very tolerant of pollution. The Ginkgo tree, like the cycads, has separate female 
and male plants. The male trees are usually preferred for landscaping because the seeds produced by the 
female plants smell rather foul as they ripen. 



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Figure 6: Ginkgo trees are gymnosperms with broad leaves. 

(Source: http://www.flickr.com/photos/paranoid-android_arg/222571 9000/, License: CC-Attribution) 

The Gnetophytes, in the phylum Gnetophyta, are a very small and unusual group of plants. Ephedra is an 
important member of this group since this desert shrub produces the ephedrine used to treat asthma and 
other conditions. Welwitschia produces extremely long leaves and is found in the deserts of southwestern 
Africa (Figure 7). Overall, there are about 70 different species in this very diverse phylum. 




Figure 7: One type of gnetophyte is Welwitschia. 

(Source: http://www.flickr.com/photos/squeakymarmot/134394329/, License: CC-Attribution) 

Angiosperms 

Angiosperms, in the phylum Anthophyta, are the most successful phylum of plants and vastly outnumber 
the individuals in other phyla. The feature that distinguishes the angiosperms is the evolution of the flower, 
so they are also called the flowering plants. Angiosperms inhabit a variety of environments; a water lily, an 
oak tree, and a barrel cactus are all angiosperms. 



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Figure 8: Angiosperms are the flowering plants. 

(Source: http://www.flickr.eom/photos/21857545@N02/2371488421/, License: CC-Attribution) 

Even though flowers may differ widely in their appearance, they do have some structures in common. The 
outermost structure is the sepals, collectively known as the calyx, which are usually green and protect the 
flower before it opens. The petals, collectively known as the corolla, are often bright and colorful to attract 
a particular pollinator, an animal that carries pollen from one flower to another. The next structure is the 
stamens, consisting of the stalk-like filament that holds up the anther, or the pollen sacs. The pollen is the 
male gametophyte. At the very center is the carpel, which is divided into three different regions: the sticky, 
knob-like stigma where the pollen lands, the slender tube of the style, and the enlarge base known as the 
ovary. The ovary is where the ovules, the female gametophytes, are found. When the ovules are fertilized, 
the ovule becomes the seed and the ovary becomes the fruit. Some flowers have all these parts and are 
known as complete flowers, while others may be missing one or more of these parts and are known as 
incomplete flowers. 



Stigma 
Style 
filament 



.Mature Flower 



!« <H"' 




/v_/ Anther 
Micros poranglum 



Stamen 



Figure 9: A complete flower has sepals, petals, stamens, and one or more carpels. 

(Source: http://commons.wikimedia.org/wiki/lmage: Mature_flower_diagram.svg, License: Public Domain) 



Flower 
part 



Definition 



sepals 



Outermost layer of the flower that is usually leaf-like and green. 



calyx 



The sepals collectively; outermost layer of the flower. 



228 



corolla 



The petals of a flower collectively. 



stamens 



The part of the flower consisting of a filament and an anther that produces pollen. 



filament 



Stalk that holds up the anther. 



anther 



The pollen-containing structure in a flower. 



carpel 



'Female" portion of the flower; consists of stigma, style, and ovary. 



stigma 



The knoblike section of the carpel where the pollen must land for fertilization to occur. 



style 



Slender tube that makes up part of the carpel. 



ovary 



Enlarged part of the carpel where the ovules are contained. 



Figure 10: Review the parts of the flower. 

(Source: Jessica Harwood, License: CC-BY-SA) 

Many plants can self-pollinate, meaning that pollen falls on the stigma of the same flower. Cross-fertilization 
is often favored and occurs when the pollen from an anther is transferred to a stigma of another flower on 
another plant. This can be accomplished two ways, by wind or by animals. Flowers that are pollinated by 
animals such as birds, butterflies, or bees are often colorful and provide nectar, a sugary reward, for their 
animal pollinators. 

Angiosperms are important to humankind in many ways, but the most significant role of angiosperms is as 
food. Wheat, rye, corn, and other grains are all harvested from flowering plants. Starchy foods, such as 
potatoes, and legumes, such as beans, are also angiosperms. And as mentioned previously, fruits are a 
product of angiosperms to increase seed dispersal and are also nutritious foods. There are also many non- 
food uses of angiosperms that are important to society; for example, cotton and other plants are used make 
cloth, and hardwood trees to make lumber. The flowering plants are dominant in the environment and are 
important resources for humans and all animals. 

Lesson Summary 

Seeds consist of a dormant plant embryo and stored food. 

Seeds can be dispersed by wind or by animals that eat fleshy fruits. 

Gymnosperms, seed plants without flowers, include the Conifers, the Cycads, the Gingko tree, and the 
Gnetophytes. 

Angiosperms are flowering plants. 

Seed plants provide many foods and products for humans. 

Review Questions 

1 . Why are seeds an adaptive feature for seed plants? (Challenging) 

2. What is the purpose of a plant developing a fruit? (Challenging) 

3. What are two ways that plants disperse their seeds? (Beginning) 

4. How do Gymnosperms and Angiosperms differ? (Intermediate) 

5. What are some examples of Gymnosperms? (Beginning) 

6. What are some uses that seed plants have for humans? (Beginning) 

7. Firs, spruces, and pines belong to what group of Gymnosperms? (Intermediate) 

8. Why is the Ginkgo tree considered a "living fossil"? (Intermediate) 



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9. Where is the pollen stored in a flower? (Beginning) 
1Q How are plants pollinated? (Intermediate) 

Further Reading I Supplemental Links 

http://home.manhattan.edu/~frances.cardillo/plants/intro/plantmen.html 

http://www.ucmp.berkeley.edu/seedplants/seedplants.html 

http://hcs.osu.edu/hcs300/gymno.htm 

http://biology.clc.uc.edu/Courses/bio106/gymnospr.htm 

http://www.biologie.uni-hamburg.de/b-online/e02/02d.htm 

http://home.manhattan.edu/~frances.cardillo/plants/intro/plantmen.html 

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookflowers.html 

http://en.wikipedia.org/wiki 

Vocabulary 

angiosperms Another name for flowering plants. 

anther The pollen-containing structure in a flower. 

calyx The sepals collectively; outermost layer of the flower. 

carpel "Female" portion of the flower; consists of stigma, style, and ovary. 

complete flowers Flowers that contain all four structures: sepals, petals, stamens, and one or more 
carpels. 

conifers Group of gymnosperms that bear cones; includes spruces, pine, and fir trees. 

corolla The petals of a flower collectively are known as the corolla. 

dormant Halting growth and development temporarily. 

ginkgo Tree known as the "living fossil" because it is the only species left in the phylum 

Ginkgophyta. 

gnetophytes Diverse group of gymnosperms that includes Ephedra and Welwitschia. 

gymnosperms Seed plants in which the seeds are not encased in a fruit. 

incomplete flowers Flowers that are missing one or more structures: sepals, petals, stamens, or carpels. 

ovary Enlarged part of the carpel where the ovules are contained. 

sepals Outermost layer of the flower that is usually leaf-like and green. 

stamens The part of the flower consisting of a filament and an anther that produces pollen. 

stigma The knoblike section of the carpel where the pollen must land for fertilization to occur. 

Review Answers 

1 . They provide food for the embryo, and the embryo can remain dormant until favorable conditions arrive. 

2. Fruits are eaten by animals, which disperse the seeds. 

3. by wind or animals 

4. Gymnosperms have "naked" seeds, while only angiosperms form fruits around their seeds. Also, only 
angiosperms produce flowers. 

5. conifers, ginkgo trees, cycads, gnetophytes 
230 



6. food, paper, lumber, cloth, etc. 

7. conifers 

8. The Ginkgo tree is the only remaining living species in its phylum. 

9. the anther (of the stamen) 

1Q Through wind or by animals carrying pollen from flower to flower. 

Points to Consider 

• Do you think plants can sense their environment? Why or why not? 

• Can you think of an example of a hormone? 

• Do you think that plants have hormones? 

• How do you think trees know when it's time to lose their leaves? 

Plant Responses 

Lesson Objectives 

• List the major types of plant hormones and the main functions of each. 

• Define tropism and explain examples of tropisms. 

• Explain how plants detect the change of seasons. 

Check Your Understanding 

• Why do pants need sunlight? 

Introduction 

Plants may not move, but that does not mean they don't respond to their environment. Plants are constantly 
responding to their surroundings. Plants detect and respond to stimuli such as gravity, light, touch, and 
seasonal changes. For example, you might have noticed how a house plant bends towards a bright window. 
Plants must be able to detect and respond to changes in the direction of light. You probably also have noticed 
that some trees lose their leaves in the autumn, so plants must be able to detect the time of year. Plants 
are able to respond to stimuli through the help of special chemical messengers, called hormones. The 
various ways that plants respond to their environment and the hormones that control these responses will 
be the focus of this section. 

Plant Hormones 

In order for plants to respond to the environment, their cells must be able to communicate with other cells. 
The chemical signals that travel through cells to help them communicate are called hormones. You might 
be familiar with the term hormones since animals, including humans, also depend on hormones, such as 
testosterone or estrogen, to carry messages from cell to cell. Animal hormones will be discussed in the 
Controlling the Body chapter. In both plants and animals, hormones travel from cell to cell in response to a 
stimulus and also activate a specific response. 



Hormone 



Ethylene 



Function 



Fruit ripening and abscission 



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Gibberellins 


Break the dormancy of seeds and buds; promote growth 


Cytokinins 


Promote cell division; prevent senescence 


A b s c i s i c 
Acid 


Close the stomata ; maintain dormancy 


Auxins 


Involved in tropisms and apical dominance 



Table 1: Each plant hormone has a specific function. 

(Source: Jessica Harwood, License: CC-BY-SA) 

Ethylene is the plant hormone involved in ripening fruit and with abscission, the dropping of leaves, fruits 
and flowers. When a flower is done blooming or a fruit is ripe and ready to be eaten, ethylene stimulates 
the production of enzymes that allow the petals or fruit to separate from the plant (Figures 1 and 2). Ethylene 
is an unusual plant hormone because it is a gas. That means it can move through the air, and a ripening 
apple can cause another to ripen, or even over-ripen. That's why one rotten apple spoils the whole barrel! 




Figure 1: The hormone ethylene is signaling these tomatoes to ripen. 

(Source: http://www.flickr.com/photos/starmist1/794903253/, License: CC-Attribution) 




Figure 2: The hormone ethylene plays a role in signaling these flower petals to separate and drop, a process 
known as abscission. 

(Source: http://www.flickr.com/photos/sararah/2516062323, License: CC-SA) 



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Gibberellins are growth-promoting hormones. When gibberellins are applied artificially to plants, the stems 
grow longer. Therefore, gibberellins can be used in horticulture to increase the growth of ornamental plants, 
whereas dwarf plants have low concentrations of gibberellins (Figure 3). Another function of gibberellins is 
to break the dormancy of seeds and buds. Gibberellins signal that it's time for a seed to germinate or for a 
bud to open. 




Figure 3: Dwarf plants like this bonsai tree often have unusually low concentrations of gibberellins. 

(Source: http://www.flickr.com/photos/sheilaellen/113416073/, License: CC-Attribution) 

Cytokinins are hormones that promote cell division. Cytokinins were discovered from attempts to grow 
plant tissue in artificial media (Figure 4). Cytokinins also prevent senescence, the programmed aging process. 
As a result, florists sometimes apply cytokinins to cut flowers. 




Figure 4: Cytokinins promote cell division and are necessary for growing plants in tissue culture; a small 
piece of a plant is placed in sterile conditions to regenerate a new plant. 

(Source: http://www.flickr.com/photos/ricephotos/367696112/, License: CC-SA) 

Abscisic Acid is misnamed because it was once believed to play a role in abscission (the dropping of 
leaves, fruits and flowers), but we now know abscission is regulated by ethylene. The actual role of abscisic 
acid is to close the stomata and maintain dormancy. When a plant is stressed due to lack of water, abscisic 
acid signals the stomata to close. This prevents excess water loss through the stomata. When conditions 
are not ideal for a seed to germinate or for a winter bud to put out its leaves, abscisic acid signals for dormancy 
to continue. When conditions improve, the levels of abscisic acid drop and the levels of gibberellins increase, 
signaling that is time to break dormancy (Figure 5). 



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Figure 5: A reduction in levels of abscisic acid allows these buds to break dormancy and put out leaves. 

(Source: http://www.flickr.com/photos/martinlabar/529695331/, License: CC-Attribution) 

Auxins are hormones that influence many different processes in plants. Auxins produced at the tip of the 
plant are involved in apical dominance, preventing the growth of side branches. In apical dominance the 
main central stem of the plant is dominant over other side stems; the main stem grows more strongly than 
other stems and branches. When the tip of the plant is removed, the auxins are no longer present and the 
side branches begin to grow. This is why pruning generally will help produce a fuller plant with more branches. 
Auxins are also involved in tropisms, which will be discussed in the next section. 

Tropisms 

Plants may not be able to move, but they are able to change their growth in response to a stimulus. Growth 
toward or away from a stimulus is known as a tropism. The ability of a plant to curve its growth in one direction 
is achieved with the signaling of auxin. The auxin moves to one side of the stem, where it starts a chain of 
events that elongate the cells on just that one side of the stem. With one side of the stem growing faster 
than the other, the plant begins to bend. 

You might have noticed that plants tend to bend towards the light. This is an example of a tropism where 
light is the stimulus, known as phototropism (Figure 6). To obtain more light for photosynthesis, it's advan- 
tageous for leaves and stems to grow towards the light. On the other hand, roots are either insensitive to 
light or actually grow away from light. This is advantageous for the roots since their purpose is to obtain 
water and nutrients from deep within the ground. 




Figure 6: These seedlings bending toward the sun are displaying phototropism. 



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(Source: http://www.flickr.com/photos/rneches/2081938105/, License: CC-Attribution) 

A seed often starts out underground in the dark, yet the roots always grow downwards into the earth and 
not toward the surface. How do the roots know which way is up? Gravitropism is a growth towards or away 
from the pull of gravity. Shoots also exhibit gravitropism, but in the opposite direction. If you place a plant 
on its side, the stem and new leaves will curve upwards. Again, the hormone auxin is involved in this response. 
Auxin builds upon the lower side of the stem, elongating this side of the stem and causing it to bend upwards 
overtime. 

Plants also have a touch response, called thigmotropism. If you have ever seen a morning glory or the 
tendrils of a bean plant twist around a pole, then you know that plants must be able to detect the pole. 
Thigmotropism works much like the other tropisms. The plant grows straight until it comes in contact with 
the pole. Then the side of the stem in contact with the pole grows slower than the opposite side of the stem. 
This causes the stem to bend around the pole. 



Type of Tropism 


Stimulus 


Phototropism 


light 


Gravitropism 


gravity 


Thigmotropism 


touch 



Table 2: Tropisms 

Seasonal Changes 

Along with detecting differences in light or gravity, plants also are able to detect the seasons. Leaves change 
color and drop each autumn in temperate climates (Figure 7). Certain flowers, like poinsettias, only bloom 
during the winter. And in the spring, the winter buds on the trees break open and the leaves start to grow. 
How do plants detect time of year? 

Although you might detect the change of seasons by the change in temperature, this is not the primary way 
by which plants detect the change of seasons. Plants determine the time of year by the length of the day. 
Because of the tilt of the Earth, during winter days there are less hours of light than during summer days. 
That's why during the winter it may start getting dark very early during the evening and even stay dark while 
you're getting ready for school the next morning. But in the summer it will be bright early in the morning and 
the sun may not set until late that night. Plants can detect the differences in day length and respond accord- 
ingly. For example, in the fall when the days start to get shorter, the trees sense it is time to begin the process 
of shedding their leaves. 




Figure 7: Leaves changing color is a response to the shortened length of the day in autumn. 



235 



(Source: http://www.flickr.com/photos/wandererwoman/21801215/, License: CC-Attribution) 

Lesson Summary 

• Plant hormones are chemical signals that regulate a variety of processes in plants. 

• A plant tropism is growth towards or away from a stimulus such as light or gravity. 

• Many plants undergo seasonal changes after detecting differences in day length. 

Review Questions 

1 . What is the term for dropping fruits, flowers, or leaves? (Beginning) 

2. What hormone is involved with fruit ripening? (Beginning) 

3. How are hormones involved in seed germination? (Challenging) 

4. What hormone is involved in tropisms? (Beginning) 

5. What hormones promote cell division? (Beginning) 

6. What hormone causes stems to elongate? (Beginning) 

7. What is phototropism? (Intermediate) 

8. How does a tendril wind around a pole? (Challenging) 

9. How do plants detect the change in seasons? (Beginning) 

1Q What are some seasonal responses in plants? (Intermediate) 

Further Reading I Supplemental Links 

www.plantphysiol.org/cgi/reprint/11 6/1 /329.pdf 

http://plantphys.info/apical/apical.html 

http://www.cals.ncsu.edu/nscort/outreach_exp_gravitrop.html 

http://biology.kenyon.edu/edwards/project/steffan/b45sv.htm 

http://www.bbc.co.uk/schools/gcsebitesize/biology/greenplantsasorganisms/2plantgrowthrev1.shtml 

http://en.wikipedia.org/wiki 

Vocabulary 

abscisic acid Plant hormone involved in maintaining dormancy and closing the stomata. 

abscission The shedding of leaves, fruits, or flowers. 

apical dominance Suppressing the growth of the side branches of a plant. 

auxin Plant hormone involved in tropisms and apical dominance. 

cytokinins Plant hormone involved in cell division. 

ethylene Plant hormone involved in fruit ripening and abscission. 

gibberellins Plant hormone involved in seed germination and stem elongation. 

gravitropism Plant growth towards or away from the pull of gravity. 

hormones Chemical messengers that signal responses to stimuli. 



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phototropism Plant growth towards or away from light. 

senescence The programmed process of aging and eventual death. 

thigmotropism Differential plant growth in response to contact with an object. 

tropism Plant growth response towards or away from a stimulus. 

Review Answers 

1. abscission 

2. ethylene 

3. Increasing levels of gibberellins and lower levels of abscisic acid trigger germination. 

4. auxin 

5. cytokinins 

6. gibberellins 

7. Differential growth in response to light, causing growth toward or away from light. 

8. Thigmotropism; the side of the stem in contact with the pole grows slower than the opposite side of the 
stem. 

9. They sense the length of the day. 

1Q flowering, breaking bud dormancy, dropping leaves 

Points to Consider 

In the next chapter we will turn our attention to animals. 

• List some ways animals are different from plants. 

• What characteristics do you think define an animal? 

• Can you think of examples of animals that do not have hard skeletons? 



237 



238 



11. Introduction to Invertebrates 



Overview of Animals 

Lesson Objectives 

• List the characteristics that define the animal kingdom. 

• Define and give examples of the invertebrates. 

Check Your Understanding 

• What are the main differences between an animal cell and a plant cell? 

• How do animals get their energy? 

introduction 

How are animals different from other forms of life? Recall that all animals are eukaryotic, meaning that they 
have cells with true nuclei and membrane-bound organelles. Another feature that distinguishes animals 
from animal-like protists is that animals are multicellular, while protists are often unicellular. Because animals 
are multicellular, animal cells can be organized into tissues, organs, and organ systems. Finally, animals 
are heterotrophic, meaning they must ingest some type of organic matter for nutrition and energy. 

Eukaryotic, multicellular, and heterotrophic are features shared by all the millions of diverse types of animals 
on earth, from tiny ants and snails to giant whales and grizzly bears. In this chapter we will just focus on the 
invertebrates, the animals that do not have a backbone of bone or cartilage. 




Figure 1 : Animals are heterotrophs, meaning they must eat to get molecules necessary for their growth and 
energy. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/2/2f/Turdus_migratorius_4480.jpg, License: 
GNU_FD, CC_SA) 

Classification of Animals 

Recall that each kingdom of life, including the animal kingdom, is divided into smaller groups called phyla 
based on their shared characteristics. For example the phylum Mollusca largely consists of animals with 
shells like snails and clams. Although modern classification is also based on looking at molecular data, such 



239 



as DNA sequencing, animals have long been classified in their current phyla largely by their physical char- 
acteristics. 

One example of a physical characteristic used to classify animals is body symmetry. In radially symmetrical 
organisms, such as sea stars, the body is organized like a circle (Figure 2). Therefore, any cut through the 
center of the animal results in two identical halves. Other animals, such as humans and worms, are bilaterally 
symmetrical, meaning their left and right sides are mirror images. 




Figure 2: Sea stars are radially symmetrical. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Sea-Star-Ship-Harbor-Acadia-NP.jpg, License: CCSA) 

Animals are also often classified by their body structure. For example, segmentation, the repetition of body 
parts, defines one phylum of worms (Figure 3). Animals that have a true body cavity, defined as a fluid-filled 
space, and internal organs are also classified in separate phyla from those animals that do not have a true 
body cavity. Finally, the structure of the digestive system of animals can also be used as a characteristic 
for classification. Animals with incomplete digestive tracts have only one opening in their digestive tracts, 
while animals with complete digestive tract have two openings, the mouth and anus. 




Figure 3: A segmented body plan defines the phylum that includes the earthworms. 

(Source: http://www.flickr.com/photos/rastafabi/747108603/, License: CC_Attribution) 

What Are Invertebrates? 

Besides being classified into phyla, animals are also often characterized as being invertebrates or vertebrates. 
This is an informal classification term based on the skeletons of the animals. Vertebrates have a backbone 
of bone or cartilage, while invertebrates have no backbone. All vertebrate organisms are in the phylum 
Chordata, while invertebrates make up several diverse phyla. As seen in figure 5, the invertebrates include 
the insects, the earthworms, the jellyfish, the star fish, and a variety of other animals. In the next lessons 
we will discuss some of phyla within the animal kingdom that contain invertebrates. 



240 




Figure 4: Snails are an example of invertebrates, animals without a backbone. 

(Source: http://www.flickr.eom/photos/14376024@N00/178810773/, License: CC_Attribution) 



Phylum 


Examples 


Porifera 


Sponges 


Cnidaria 


Jellyfish, corals 


P I a t y - 
helminthes 


Flatworms, tapeworms 


Nematoda 


Nematodes, heartworm 


Mollusca 


Snails, clams 


Annelida 


Earthworms, leeches 


Arthropoda 


Insects, crabs 


Echinodermata 


Sea stars, sea urchins 



Figure 5: The invertebrates, animals without a backbone, include a variety of organisms. 
(Source: Jessica Harwood, Permissions: Jessica Harwood) 

Lesson Summary 

• Animals are multicellular, eukaryotic heterotrophs. 

• Animals can be classified by both molecular data and physical characteristics such as symmetry. 

• Invertebrates are animals without a backbone. 

Review Questions 

1 . What are some key features that define the animal kingdom? (Intermediate) 

2. What does heterotrophic mean? (Beginning) 

3. What defines the invertebrates? (Beginning) 

4. What are some examples of invertebrates? (Challenging) 

5. What is the difference between radially and bilaterally symmetrical animals? (Challenging) 

6. What's an example of a bilaterally symmetrical animal? (Intermediate) 



241 



7. What are some examples of a radially symmetrical animal? (Intermediate) 

8. What is a body cavity? (Beginning) 

9. What is the difference between an incomplete and complete digestive system? (Intermediate) 

10. What is segmentation? (Beginning) 
Further Reading 

http://animaldiversity.ummz.umich.edu/site/index.html 
http://doe.sd.gov/octa/ddn4learning/themeunits/animals 
http://animals.nationalgeographic.com/animals/invertebrates.html 
www.enwikipedia.com 

Vocabulary 

bilaterally symmetrical Body plan in which the left and right side are mirror images. 

complete digestive tract A digestive tract that has two openings, the mouth and the anus. 

heterotroph Organism that cannot make its own food, so it must ingest some type of or- 

ganic matter. 

invertebrates Animals without a backbone. 

incomplete digestive tract A digestive tract that has only one opening. 

radially symmetrical A body plan in which any cut through the center results in two identical halves. 

segmentation Repetition of body parts or segments. 

Review Answers 

1. Eukaryotic, multicellular, heterotrophic 

2. Must ingest and break down organic material for energy and nutrients 

3. No backbone 

4. Sponges, jellyfish, flatworms, earthworms, snails, crayfish 

5. Bilaterally symmetrical means having mirror imaged left and right sides; radially symmetrical means any 
cut through a center resulting in identical halves 

6. People, fish, cats, dogs 

7. Sea star, jellyfish, coral 

8. Space that contains the internal organs 

9. Incomplete has only one opening, while complete has two, mouth and anus 

10. Repetition of body parts 

Points to Consider 

• What do you think that jellyfishes and corals have in common? 



242 



Think of some examples of animals that are bilaterally symmetrical, where the left side is a mirror image 
of the right? 



Sponges and Cnidarians 

Lesson Objectives 

• Describe the key features of the Sponges. 

• Describe the key features of the Cnidarians. 

• List examples of the Cnidarians. 

Check Your Understanding 

• How are animals classified? 

• What is an invertebrate? 

Introduction 

The ocean is home to a variety of organisms. Phytoplankton, tiny photosynthetic organisms that float in the 
water, make their own food from the energy of the sun. Small aquatic animals, known as zooplankton, and 
larger animals, such as fish, use phytoplankton as a food source. These animals can in turn be eaten by 
larger aquatic animals, such as larger fish and sharks. 

Among the various types of animals that live in the ocean, the sponges and cnidarians are important inver- 
tebrates. The Sponges are believed to be one of the most ancient forms of animal life on earth. The 
cnidarians, which include the jellyfish, also are among the oldest and most unusual animals on earth. In this 
lesson we will discuss the features that make these two types of invertebrates unique from other types of 
animals. 

Sponges 

Sponges are classified in the phylum Porifera, which derives its name from Latin words meaning "pore 
bearing." These pores allow the movement of water into the sponges' saclike bodies (Figure 1). Sponges 
pump water through their bodies because they are sessile filter feeders, meaning they cannot move and 
must filter organic matter and tiny organisms out of the water to obtain food. 







Mtild 


5* * fc*t 


■ - jj 


r 


IPS ■ 


L 


X>-s J 




1 1 


■ ■ 


* 


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Figure 1: Sponges have tube-like bodies with many pores. 

243 



(Source: http://www.flickr.com/photos/icelight/208857632/, License: CC_Attribution) 

Sponges are relatively primitive animals and do not have brains, stomachs, or other organs. In fact, sponges 
do not even have true tissues. Instead, their bodies are made up of specialized cells that each has specific 
functions. For example, the collar cells are flagellated and encourage water movement, while other types 
of cells regulate the water flow by increasing or decreasing the size of the pores. 

Cnidarians 

The cnidarians, in the phylum Cnidaria, include organisms such as the jellyfish (Figure 2) and sea anemones 
(Figures 3&4) that are found in shallow ocean water. You might recognize that these animals can give you 
a painful sting if you step on them. That's because cnidarians have stinging cells known as nematocysts. 
When touched, the nematocysts unleash long, hollow threads that are intended to trap prey, and sometimes 
toxins are also injected through these threads to paralyze the prey. 

The body plan of cnidarians is unique because these organisms are radially symmetrical, meaning that they 
have a circular body plan so that any cut through the center of the animal leaves two equal halves. The 
cnidarians have two basic body forms, polyp and medusa. The polyp is a cup-shaped body with the mouth 
directed upward, such as a sea anemone (Figure 3). The medusa is a bell-shaped body with the mouth 
and tentacles directed downward, such as a jellyfish (Figure 2). 

Unlike the sponges, the cnidarians are made up of true tissues. The inner tissue layer secretes digestive 
enzymes into the gastrovascular cavity, a large cavity that has both digestive and circulatory functions. 
The cnidarians also have nerve tissue organized into a net-like structure. Cnidarians do not have true organs, 
however. 




Figure 2: Jellyfish have bell-shaped bodies with tenticles. 

(Source: http://www.flickr.com/photos/pierreyves0/2438937060/, License: CC_Attribution) 



244 




Figure 3: Sea anemones can sting and trap fish with their tentacles. 

(Source: http://www.flickr.com/photos/pierreyves0/2438937060/, License: CC_Attribution) 




Figure 4: One type of sea anemone is home to the clownfish. 

(Source: http://www.flickr.com/photos/cybersam/1004710143/, License: CC_Attribution) 

Cnidarian Colonies 

Some types of cnidarians are also known to form colonies. For example, the Portuguese man-of-war looks 
like a single organism but is actually a colony of polyps (Figure 5). One polyp is filled with air to help the 
colony float, while several feeding polyps hang below with tentacles full of nematocysts. Consequently, the 
Portuguese man-of-war is known to cause extremely painful stings to swimmers and surfers who accidentally 
brush up against these creatures in the water. 




245 



Figure 5: The Portuguese man-o-war can deliver debilitating stings with its tentacles. 

(Source: http://www.flickr.com/photos/wimi/378691686/, License: CC_Attribution) 

Coral reefs are built from colonial cnidarians called corals (Figure 6). The corals are sessile polyps that can 
extend their tentacles to feed on ocean creatures that pass by. Their skeletons are made up of calcium 
carbonate, which is also known as limestone. Over long periods of time, their skeletons can accumulate to 
produce massive structures known as coral reefs. Coral reefs are important habitats for diverse types of 
ocean life. 




Figure 6: Corals are colonial cnidarians. 

(Source: http://www.flickr.com/photos/laszlo-photo/278151588/, License: CC_Attribution) 

Lesson Summary 

• Sponges are sessile filter feeders without true tissues. 

• The cnidarians, such as jellyfish, are radially symmetrical with true tissues. 

• Colonial cnidarians include the Portuguese man-of-war and corals. 

Review Questions 

1 . What is the only animal to lack true tissues? (Beginning) 

2. In what phylum are the sponges? (Beginning) 

3. How do sponges gain nutrition? (Intermediate) 

4. Cnidarians are radially symmetrical. What does this mean? (Challenging) 

5. What are some examples of cnidarians? (Challenging) 

6. How do cnidarians sting their prey? (Intermediate) 

7. Describe the nervous system of the cnidarians. (Intermediate) 

8. How is a jellyfish different from a Portuguese man-o-war? (Intermediate) 

9. How are coral reefs built? (Beginning) 

246 



10. Where are most cnidarians found? (Beginning) 
Further Reading I Supplemental Links 

http://www.ucmp.berkeley.edu/porifera/porifera.html 

http://animaldiversity.ummz.umich.edu 

http://www.pbs.org/kcet/shapeoflife/animals/cnidaria.html 

http://tolweb.org/tree?group=Cnidaria&contgroup=Animals http://www.ucmp. berke- 

ley.edu/cnidaria/cnidaria.html 

http://animaldiversity.ummz.umich.edu/site/accounts/information/Porifera.html 

http://www.pbs.org/kcet/shapeoflife/animals/cnidaria.html 

http://en.wikipedia.org/wiki/Cnidaria 

Vocabulary 

corals Cnidarians that live on ocean reefs in colonies. 

cnidarians Invertebrates that have radial symmetry and include the jellyfish. 

filter feeders An organism that feeds by filtering organic matter out of water. 

gastrovascular cavity A large cavity having both digestive and circulatory functions. 

medusa Cnidarian with a bell-shaped body directed downward. 

nematocysts Specialized cells in cnidarians that can release a small thread-like structure and 

toxins to capture prey. 

porifera Filter-feeders with saclike bodies; known as the sponges. 

polyp Cnidarian with a cup-shaped body directed upward. 

sessile Permanently attached and not freely moving. 

Review Answers 

1. Sponges 

2. Phylum Porifera 

3. Filter feeders; they filter the water for small organisms and organic matter 

4. No left or right side; any cut through the center results in two identical halves 

5. Jellyfish, sea anemones, coral 

6. Stinging cells called nematocysts can discharge threads that inject toxins 

7. Nerve net; no central nervous system 

8. The Portuguese man-o-war is a colony while the jellyfish is a single organism 

9. From the skeletons of the corals 



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10. Shallow ocean waters 

Points to Consider 

• How do you think that worms are different from sponges and cnidarians? 

• How do you think that worms might be similar to sponges and cnidarians? 

Worms 

Lesson Objectives 

• Describe the major features of the flatworms. 

• Describe the major features of the roundworms. 

• Describe the major features of the segmented worms. 

Check Your Understanding 

• In terms of body structure, what does segmentation refer to? 

• What is a body cavity? 

Introduction 

Calling an animal a worm is an informal, non-scientific classification for animals that have long bodies with 
no appendages. Worms are bilaterally symmetrical, meaning that the right side of their bodies is a mirror of 
the left. Worms live in a variety of environments, including in the ocean, in fresh water, on land, and as 
parasites of plants and animals. 

In this chapter we will discuss three types of worms: the flatworms, the roundworms, and the segmented 
worms. These worms are distinguished from each other by their body plan. The flatworms have flat ribbon- 
like bodies with no body cavity. The roundworms have a body cavity but no segments. The segmented 
worms have both a body cavity and segmented bodies. 

Flatworms 

Worms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. Some 
species of flatworms are free live-living organisms that feed on small organisms and decaying matter. These 
types of flatworms include marine flatworms and fresh-water flatworms such as Dugesia (Figures 1&2). 
Other types of flatworms are parasitic and rely on a host organism for energy. For example, tapeworms 
have a modified head region with tiny hooks that help the worm attach to the intestines of a animal host 
(Figures 3&4). 



248 




Figure 1: Dugesia is a type of flatworm with a head region and eyespots. 

(Source: http://commons.wikimedia.org/wiki/lmage: Planariafull.jpg, License: Public Domain) 







^ 


h.- .' 




y 




;- a 


• 


' ><*.. >4 i r -~ p V a *rtk> _■ ^ .'. HChjI&mB.I 



Figure 2: Marine flatworms can be brightly colored. 

(Source: http://www.flickr.com/photos/steve_childs/233603352/, License: CC_Attribution) 




Figure 3: Tapeworms are parasitic flatworms that live in the intestines of their hosts. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Taenia_saginata_adult_5260_lores.jpg, License: Public 
Domain) 



249 




Figure 4: Tapeworms attach to the intestinal wall with a head region that has hooks and suckers. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Tenia_solium_scolex.jpg, License: Public Domain) 

Flatworms have no true body cavity and an incomplete digestive system, meaning that the digestive tract 
has only one opening. Flatworms do not have a respiratory system, so gas exchange occurs at surface of 
their bodies. Furthermore, there are no blood vessels or true circulatory system in the flatworms. Their 
gastrovascular cavity serves for both digestion and to distribute nutrients. The flatworms do have a ladder- 
like nervous system with a distinct head region with a concentration of nerve cells and sensory organs such 
aseyespots (Figure 1). The development of a head region, called cephalization, arose with the development 
of bilateral symmetry in animals. 

Roundworms 

The phylum Nematoda includes non-segmented worms known as nematodes or roundworms (Figure 5). 
Unlike the flatworms, the roundworms have a body cavity with internal organs. A roundworm's complete 
digestive tract, meaning the digestive tract includes both a mouth and anus, includes a large digestive organ 
known as the gut. Roundworms also have a simple nervous system with a primitive brain. Both their anterior 
and posterior ends have specialized sensory nerves. These nerves are connected with a ventral and dorsal 
nerve cord that run the length of the body. 




Figure 5: Nematodes can be parasites of plants and animals. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Meloidogyne_incognita.jpg, License: Public Domain) 



250 



Roundworms can be free-living organisms, but they are probably best known for their role as significant 
plant and animal parasites. The heartworms, which cause serious disease in dogs while living in the heart 
and blood vessels, are a type of roundworm. Round worms can also cause disease in humans. Elephantiasis, 
a disease characterized by the extreme swelling of the limbs, is caused by infection with a type of roundworm 
(Figure 6). 




Figure 6: One roundworm parasite causes elephantiasis, a disease characterized by the swelling of the 
limbs. 

(Source: http://www.flickr.eom/photos/27920559@N05/2778292954/, License: CC_Attribution) 
Segmented Worms 

The phylum Annelida includes the segmented worms such as the common earthworm, some marine worms, 
and leeches (Figures 7 & 8). These worms are known as the segmented worms because their bodies are 
segmented, or separated into repeating units. Most segmented worms feed on dead organic matter, while 
leeches can live in freshwater and suck blood from host organisms. Leeches can also be used medicinally 
to remove excess blood. 




Figure 7: Earthworms are segmented worms. 

(Source: http://www.flickr.com/photos/squeezyboy/82103077/, License: CC_Attribution) 



251 




Figure 8: Leeches are parasitic segmented worms. 

(Source: http://www.flickr.com/photos/robandstephanielevy/221 2070602/, License: CC_Attribution) 

Segmented worms have a well-developed body cavity filled with fluid, which serves as a hydroskeleton, a 
supportive structure that aids in muscle contraction. Segmented worms also tend to have organ systems 
that are more developed than the roundworms or flatworms. Earthworms, for example, have a complete 
digestive tract including an esophagus and intestines. The circulatory system consists of paired hearts and 
blood vessels, while the nervous system consists of the brain and a ventral nerve cord. 



Type of Worm 


Body Cavity 


Segmented 


Digestive System 


Example 


Flatworm 


No 


No 


Incomplete 


Tape- 
worm 


Roundworm 


Yes 


No 


Complete 


Heart- 
worm 


Segmented 


Yes 


Yes 


Complete 


Earth- 
worm 



Figure 9: Comparison of the three phyla containing worms. 
(Source: Jessica Harwood, License: CC-BY-SA) 

Lesson Summary 

• The flatworms have no true body cavity and include free-living Dugesia and parasitic tapeworms. 

• The roundworms, which can also be parasitic or free-living, are non-segmented worms with a complete 
digestive tract and a primitive brain. 

• The segmented worms include the common earthworm and leeches. 
Review Questions 

1. Are all worms classified into a single phylum? (Intermediate) 

2. Describe the respiratory system of the flatworms. (Challenging) 

3. What is cephalization? (Challenging) 

4. Name a parasitic flatworm. (Beginning) 



252 



5. How does the body plan of the roundworms differ from that of the flatworms? (Beginning) 

6. Describe the digestive system of roundworms. (Intermediate) 

7. What features distinguish Phylum Annelida from the other worms? (Intermediate) 

8. Describe the skeletal system of a segmented worm. (Intermediate) 

9. Name a parasitic segmented worm. (Beginning) 

10. Earthworms are in what phylum? (Intermediate) 
Further Reading I Supplemental Links 

http://animaldiversity.ummz.umich.edu/site/accounts/information/Annelida.html 
http://animaldiversity.ummz.umich.edu/site/accounts/information/Nematoda.html 
http://animaldiversity.ummz.umich.edu/site/accounts/information/Platyhelminthes.html 
http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthtml 
http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html 
http://www.ucmp.berkeley.edu/annelida/annelida.html 
http://animaldiversity.ummz.umich.edu 
http://en.wikipedia.org/wiki/Annelida 

Vocabulary 

annelida Invertebrate worms that have segmented bodies, such as earthworms. 

cephalization Having a head region with a concentration of sensory organs and central 

nervous system. 

complete digestive tract A digestive tract with two openings, a mouth and anus. 

gastrovascular cavity A large cavity having both digestive and circulatory functions. 

hydroskeleton Fluid-filled body cavity that provides support for muscle contraction. 

incomplete digestive system A digestive tract with only one opening. 

nematoda Invertebrate worms that include the roundworms. 

platyhelmenthes Invertebrate worms that include the flatworms and tapeworms. 

segmentation A body plan that has repeated units or segments. 

tapeworms Intestinal parasites in the phylum Platyhelmenthes. 

Review Answers 

1 . No, worms are an informal group that includes several phyla 

2. There are no respiratory structures; gas exchange is at the body surface 

3. The development of a head region 

4. Tapeworms 

5. Flatworms do not have a body cavity; roundworms have a body cavity 



253 



6. Complete digestive tract with a mouth and an anus 

7. Segmented bodies, well-developed body cavity 

8. Hydroskeleton, fluid-filled body cavity 

9. A leech 

10. Annelida, the segmented worms 

Points to Consider 

• How might the vertebrates be different from the invertebrates? 

• Can you think of some examples of animals with a backbone? 

Lab 

Survey of Some Invertebrates 

In this lab you will observe some examples of the invertebrates, those animals that do not have a backbone. 
The hydras are in the phylum Cnidaria. The Dugesia are in the phylum Platyhelmenthes, the flatworms. The 
earthworm is in the phylum Annelida. 

Materials: 

compound and dissecting microscopes 

slides and cover slips 

pipettes 

watch glass 

culture of living hydra 

Dugesia 

construction paper 

preserved earthworms 

dissection kits 

Procedure: 

A. Hydra 

With a pipette, pull up some of the material from the bottom of the culture dish. Then squeeze a coupe 
drops onto a clean slide and cover with a cover slip. Observe your hydra under the microscope and sketch 
one below. 

B. Dugesia 

With a pipette, place a couple Dugesia on a clean watch glass. Observe under the dissecting microscope. 
Sketch below, labeling the eyespots, auricles, and gastrovascular cavity. 

1 . With a dark piece of paper, cover half the watch glass. Do the Dugesia seem to prefer the shade or the 
light? Movement in response to light is called phytotaxis. 



254 



C. Earthworm 

1 . Find the clitellum. What is its function? 

2. Touch the ventral side of the worm to feel the setae. What are their function? 

3. Lay the worm on the dissecting tray with the dorsal side up. Using the forceps and the scissors, carefully 
cut open the worm along a straight line from the clitellum to the mouth. Make sure to just cut the skin so you 
do not damage the internal organs. Sketch your worm below and label the following: aortic arches, crop, 
gizzard, pharynx, dorsal blood vessel, intestine, ventral nerve cord, and seminal vesicles. 



255 



256 



12. Other Invertebrates 



Mollusks 

Lesson Objectives 

• Discuss what characteristics define mollusks. 

• Describe the different types of mollusks. 

• Explain why mollusks are important. 

Check Your Understanding 

• What is an invertebrate? 

• How are animals classified? 

Introduction 

Perhaps the best example of a wide variety of attainable mollusks is along a walk on the beach. There you 
can find the calcified shells of many different types of mollusks, most typically clams, mussels, scallops, 
oysters, and snails. Another reminder of the treasures that mollusks yield up may be as close as a jewelry 
collection. There glossy pearls, mother of pearl, and abalone shells reveal some of the unique features of 
mollusks. 




Figure 1 : The beach yields a wide variety of mollusks. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Conchiglie_Seashells_OI .jpg, Photographer: Alessio 
Sbarbaro, License: CC-BY-SA-2.5) 



257 




Figure 2: Pearls being removed from oysters. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Pearl_Oysters.jpg, Photographer: Keith Pomakis, License: 
CCA-BY-SA 2.5) 




Figure 3: The inside of a bivalve, one of the mollusk classes described in "Types of Mollusks," showing 
mother of pearl. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Margaritifera_margaritifera-binnen1.jpg, Photographer: 
Tom Meijer, License: GNU-FDL) 



258 




Figure 4: Shells of marine mollusks, including abalone. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Shells_0f_marine_M0lluscl .jpg, Photographer: Mila 
Zinkova, License: GNU-FDL) 

As you learn about the different types of mollusks and their characteristics, consider how these features 
help adapt the mollusks to their living conditions. Then also admire their features and see how people's in- 
genuity has used the mollusk's design and beauty for practical and decorative purposes. 

What are Mollusks? 

Mollusks belong to the phylum Mollusca. The mollusk body is often divided into a head with eyes or tentacles, 
a muscular foot, and a mass housing the organs. In most species, the muscular foot is used for locomotion. 
Mollusks also have a mantle, a fold of the outer skin lining the shell, which in most mollusks secretes a calcium 
carbonate external shell, just like the ones you find on the beach. 

The majority of marine mollusks have a gill or gills to absorb oxygen from the water. All species have a 
complete digestive tract that begins at the mouth and runs to the anus. Many have a feeding structure, the 
radula, found only in mollusks. The radula is composed mostly of chitin, a tough, semitransparent substance 
that is the main component of the shells of crustaceans and the outer coverings of insects. Radulae range 
from structures used to scrape algae off rocks to the beaks of squid and octopuses. 

Larval development suggests a close relationship between the mollusks and other groups, notably the an- 
nelids, any of various worms or wormlike animals, including the earthworm and leech, characterized by a 
cylindrical, elongated, and segmented body. Unlike the annelids, however, mollusks lack body segmentation 
and their body shape is usually quite different, as well. 

The giant squid, which until recently had not been observed alive in its adult form, is one of the largest inver- 
tebrates. However, the colossal squid is even larger and can grow up to 46 ft. (14 m) long. The smallest 
mollusks are snails that are microscopic in size. 



259 




Figure 5: The colossal squid, one of the largest invertebrates, here measuring 30 ft (9 m) in length. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Colossal_Squid.jpg, Photographer: Benjamindancer, 
License: GNU-FDL) 

Types of Mollusks 

Within the phylum Mollusca, there are approximately 1 60,000 living species and an estimated 70,000 extinct 
species. Mollusks are typically divided into ten classes, of which two are extinct. Which classes are you 
most familiar with? 

Table 1: Living Molluscan Classes 



Molluscan Class 


Number of 
Species 


Habitat 


Features of Class/Examples 


Caudofoveata 


70 


Deep ocean 


Worm-like organisms 


Aplacophora 


250 


Deep ocean 


Worm-like organisms 


Polyplacophora 


600 


Rocky marine shorelines 


Chitons 


M n p I a - 
cophora 


11 


Deep ocean 


Limpet-like organisms 


Gastropoda 


150,000 (80% of 
living molluscan 
diversity) 


Marine (some limpets live in 
deep ocean around hot hy- 
drothermal vents), freshwater, 
and terrestrial 


Abalone, limpets, conch, nudibranchs, sea 
hares, sea butterfly, snails, and slugs 


Cephalopoda 


786 


Marine 


Most neurologically advanced of all inver- 
tebrates; include squid, octopus, cuttlefish, 
and nautilus. 


Bivalvia 


8,000 


Marine (some clams live in 
deep ocean around hot hy- 
drothermal vents) and fresh- 
water. 


Most bivalves are filter feeders (mecha- 
nism whereby suspended matter and food 
particles are strained from the water, typi- 
cally by passing the water over a special- 
ized filtering structure); bivalves include 
clams, oysters, scallops, and mussels. 


Scaphopoda 


350 


Marine 


Tusk shells 



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Figure 6: A chiton and sea anemones at a tide pool. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Chiton_and_sea_anemones_at_tide_pool.jpg, Photog- 
rapher: Mila Zinkova, License: GNU-FDL) 




Figure 7: An example of a gastropod species, the ostrich foot. 

(Source: http://commons.wikimedia.org/wiki/lmage, Photographer: Graham Bould, License: Public Domain) 



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Figure 8: A Caribbean reef squid, an example of a cephalopod. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Caribbean_reef_squid.jpg, Photographer: Jan Derk, Li- 
cense: Public Domain) 

As you can see, the majority of mollusk species live in marine environments, and many of them are found 
intertidally in the shallow subtidal zone and on the continental shelf. Freshwater species are represented in 
the bivalves and gastropods, and some gastropods, like land snails, and slugs, live on land. 

Importance of Mollusks 

Mollusks are important in a variety of ways, including as food, for decoration, in jewelry, and in scientific 
studies. They are even used as roadbed material and in calcium supplements. 

Edible species of mollusks include numerous species of clams, mussels, oysters, scallops, marine and land 
snails, squid, and octopus. Many species of mollusks, such as oysters, are farmed in order to provide addi- 
tional food sources. 




Figure 9: An oyster harvest in France. 



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(Source: http://commons.wikimedia.0rg/wiki/lmage:France_Oyster_Harvest_bordercropped.jpg, Photographer: 
Alan Hughes, License: GNU- FDL) 

Two natural products of mollusks used for decorations and jewelry are pearls and nacre, or mother of pearl. 
A pearl is the hard, round object produced within the mantle of a living shelled mollusk. Fine quality natural 
pearls have been highly valued as gemstones and objects of beauty for many centuries. The most desirable 
pearls are produced by oysters and river mussels. 

Nacre is an iridescent inner shell layer produced by some bivalves, some gastropods, and some cephalopods, 
and has been used in sheets on floors, walls, countertops, doors, and ceilings. It is also inserted into furniture; 
it can be found in buttons, watch faces, knives, guns, and jewelry; and is used as decorations on various 
musical instruments. 

Several mollusks are ideal subjects for scientific investigation, especially in the area of neurobiology. The 
giant squid has a sophisticated nervous system and a complex brain for study. The California sea slug, also 
called the California sea hare, is used in studies of learning and memory, since it has a simple nervous 
system, consisting of just a few thousand large, easily identified neurons, but also a variety of learning tasks. 

Lesson Summary 

• The mollusk body often has a head with eyes or tentacles, a muscular foot, a mass housing the organs, 
and a mantle, which secretes the external shell. 

• Other mollusk structures include a gill or gills for absorbing oxygen, a complete digestive tract, and a 
radula. 

• Mollusks are divided into ten living classes, including the familiar gastropods, cephalopods, and bivalves. 

• Mollusks live in marine and freshwater habitats, as well as on land. 

• Mollusks are important as food, for decoration, and in scientific studies. 

Review Questions 

1 . What are the main characteristics of mollusks? (Beginning) 

2. What evidence shows that mollusks and annelids are related? How are they different? (Intermediate) 

3. What habitats do marine mollusks live in? (Intermediate) 

4. What makes the California sea slug ideal for studies of learning and memory? (Intermediate) 

5. Oysters, one of the bivalve filter feeders, filter up to five liters of water per hour. Sediment, nutrients, and 
algae can cause problems in local waters, but oysters filter these pollutants and either eat them or shape 
them into small packets that are deposited on the bottom where they are harmless. When there is a high 
concentration of bacteria in the water from sewage run-off, this can make filter feeders, like clams and 
mussels, risky to eat. What do you think happens to the pollutants in this case? (Challenging) 

Further Reading I Supplemental Links 

http://www.centerofweb.com/scitech/bio_mollusks.htm 

http://www.manandmollusc.net/links_educational.html 

http://www.oceanicresearch.org/education/wonders/mollusk.html 

http://www.manandmollusc.net/links_educational.html 

http://www.manandmollusc.net/links_medicine.html 



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http://en.wikipedia.org 
Vocabulary 

chitin A tough, semitransparent substance that is the main component of the radula. 

filter feeders A mechanism whereby suspended matter and food particles are strained from the water, 
typically by passing the water over a specialized filtering structure. 

mollusca The phylum containing ten living classes of mollusks. 

nacre The iridescent inner shell layer produced by some bivalves, some gastropods, and some 

cephalopods; also known as mother of pearl. 

pearl The hard, round object produced within the mantle of a living shelled mollusk. 

radula A molluscan feeding structure, composed mostly of chitin. 

Review Answers 

1 . The body has a head with eyes or tentacles, a muscular foot, and a mass housing the organs. Mollusks 
also have a mantle and a complete digestive tract and most have a gill or gills and a radula. 

2. Larval development suggests that they have a close relationship. Annelids have body segmentation, but 
mollusks do not. 

3. Many marine mollusks are found intertidally in the shallow subtidal zone and on the continental shelf. 
Some species live in the deep ocean around hot hydrothermal vents. 

4. It has a simple nervous system, consisting of a few thousand large, easily-identified neurons, but also a 
variety of learning tasks. 

5. The bivalves concentrate the toxins from the floating microorganisms within their tissues rather than 
eliminating them. People can then get sick by eating the contaminated shellfish. 

Points to Consider 

• Many mollusks demonstrate bilateral symmetry. How do you think this differs from the radial symmetry 
evident in echinoderms, in the next lesson? 

• As we have seen, some species of mollusks live in the deep ocean around hot hydrothermal vents. In 
the next lesson we will learn that many echinoderms also live in the deep sea. What adaptations do you 
think both groups might have for living in such a unique environment? 

• Mollusks have an exoskeleton, which is primarily external and composed of calcium carbonate. As a 
result many of these are preserved in the fossil record. How do you think this compares to the type of 
skeleton that an echinoderm has and to its fossil record? 



Echinoderms 

Lesson Objectives 

• Discuss the traits of echinoderms. 

• List the types of echinoderms. 



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• Explain the roles echinoderms play. 

Check Your Understanding 

• What is meant by body symmetry? 

• What is radial symmetry? 

• What is bilateral symmetry? 

Introduction 

We're all familiar with starfish, and also maybe sea urchins and sand dollars. The radial symmetry is what 
hits us right away, a symmetry in which the body is arranged in five parts around a central axis. Much of the 
perceived beauty of this group resides in that design. Later in this lesson, learn how symmetry takes advantage 
of the animal's habitat. 




Figure 1: A starfish, showing the radial symmetry, characteristic of the echinoderms. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Reef0296.jpg, Photography: Dr. McVey, License: Public 
Domain - NOAA) 




Figure 2: Another echinoderm. a sea urchin, showing its calcareous spines. 



265 



(Source: http://commons.wikimedia.Org/wiki/lmage:Woda-3_ubt.jpeg, Photographer: Tomasz Sienicki, License: 
GNU-FDL) 




Figure 3: An echinoderm, the keyhole sand dollar. 

(Source: http://commons.wikimedia.org/wiki/lmage: Keyhole sand dollar 01.jpg, Photographer: Sharon 
Mooney, License: GNU-FDL) 

The other things that stand out, quite literally, are the calcareous (containing calcium carbonate) spines of 
the sea urchin. If you've gone snorkeling or walked on a sandy beach you've learned to most likely watch 
out for those sharp spines. Think about how this adaptation might benefit the sea urchin in terms of predation 
and colonization by other organisms. Can you think of another use of these structures? 

These and other adaptations will be explored in more detail as we examine this most fascinating group of 
invertebrates. Next time you take a walk on the beach, you'll have appreciation for these organisms and 
how they are adapted for their environment. 



What are Echinoderms? 

Echinoderms belong to the phylum Echinodermata, which contains marine animals living at all ocean 
depths. It consists of about 7,000 living species, the largest phylum without freshwater or terrestrial members. 
Also, few other groups are so abundant in the deep ocean as well as the shallower seas. 

As mentioned earlier, echinoderms are radially symmetric. In spite of their appearance, they do not have 
an external skeleton. Instead, a thin outermost skin covers an internal endoskeleton made of tiny calcified 
plates and spines, contained within tissues of the organism, and which forms a rigid support. Some groups, 
such as the sea urchins, have calcareous spines, referred to earlier, which protect the organism from predation 
and colonization by encrusting (covering or coating) organisms. The sea cucumbers also use these spines 
for locomotion. 



266 




Figure 4: An echinoderm, the giant California sea cucumber. 

(Source: http://commons.wikimedia.org/wiki/lmage:Parastichopus_californicus_2_(pfly).jpg, Photographer: 
pfly, License: CC-BY-SA-2.0) 

Echinoderms have a unique water vascular system, a network of fluid-filled canals, which function in gas 
exchange, feeding, and also in locomotion. This system allows them to function without gill slits found in 
other organisms. Echinoderms possess a very simple digestive system, often leading directly from mouth 
to anus. They also possess an open and reduced circulatory system, but no heart. Their nervous system 
consists of a modified nerve net (interconnected neurons with no central brain). 

In most species, eggs and sperm cells are released into open water, where fertilization takes place. The 
release of sperm and eggs is coordinated temporally (to occur at the same time) in some species and spatially 
(to occur within the same location) in others. Internal fertilization takes place in a few species. Some species 
even have parental care! 

Many echinoderms have amazing powers of regeneration. Some sea stars are capable of regenerating lost 
arms, and in some cases, lost arms have been observed to regenerate a second complete sea star! Sea 
cucumbers often discharge parts of their internal organs if they perceive danger. The discharged organs 
and tissues are then quickly regenerated. 

Feeding strategies vary greatly among the different groups of echinoderms. Some are passive filter-feeders, 
absorbing suspended particles from passing water; others are grazers; others are deposit feeders, which 
feed on particles of organic matter, usually in the top layer of soil, and still others are active hunters. 

Types of Echinoderms 

The echinoderms are subdivided into two major groups, the Eleutherozoa, which contains the more familiar, 
motile classes, and the Pelmatozoa, which contains the sessile (permanently attached and not freely moving) 
crinoids, including the feather stars, which have secondarily developed a free-living lifestyle. 



267 




Figure 5: This passion flower feather star is an echinoderm. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Feather_Star.jpg, Photographer: Richard Ling, License: 
GNU-FDL) 

The following table summarizes the four main classes of echinoderms present in the Eleutherozoa Group: 



Echinoderm 
Class 


Representative Organisms 


Asteroidea 


Starfish and sea daisies 


Ophiuroidea 


Brittle stars 


Echinoidea 


Sea urchins and sand dollars 


Holothuroidea 


Sea cucumbers 




Figure 6: The giant red brittle star, an ophiuroid echinoderm. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Giant_red_brittlestar.jpg, Photographer: Public Domain, 
License: NOAA) 



268 



Echinoderms are distributed all over the world at almost all depths, latitudes, and environments in the ocean. 
They are in highest diversity in reefs but are also widespread on shallow shores, around the poles (where 
crinoids are at their most abundant) and throughout the deep ocean, where bottom dwelling and burrowing 
sea cucumbers are common, sometimes accounting for up to 90% of organisms. 

While almost all echinoderms are benthic (live on the sea floor) some sea-lilies can swim at great speeds 
for brief periods of time, and a few deep-sea sea cucumbers are fully floating. Some crinoids attach themselves 
to floating logs and debris and some sea cucumbers employ a similar strategy, attaching to the sides offish. 

Roles of Echinoderms 

Echinoderms play roles both ecologically and economically. Ecologically, sea urchin grazing reduces the 
colonizing of bare rock by such organisms as mussels and barnacles, and sand dollar and sea cucumber 
burrowing provides more oxygen at greater depths of the sea floor, thus allowing a more complex ecological 
community to develop. In addition, starfish and brittle stars prevent the growth of algal mats on coral reefs, 
so that the coral can more effectively filter-feed. 

Echinoderms are also the staple diet of many organisms, including the otter. Many sea cucumbers provide 
a habitat for parasites, including crabs, worms, and snails. The extinction of large quantities of echinoderms 
appears to have caused a subsequent overrunning of ecosystems by seaweed, causing the destruction of 
entire reefs. 

Economically, in some countries echinoderms are regarded as delicacies. Around 50,000 tons of sea urchins 
are captured each year, and certain parts are consumed mostly in Japan, Peru, and France. Sea cucumbers 
are considered a delicacy in some southeastern Asian countries. 

Some sea cucumber toxins slow down the growth rate of tumor cells, so there is an interest in using these 
in cancer research. The calcareous external covering of echinoderms is used as a source of lime by farmers 
in some areas where limestone is unavailable and 4,000 tons of the animals are used each year for this 
purpose. 

Lesson Summary 

• Echinoderms belong to the phylum Echinodermata, the largest phylum without freshwater or terrestrial 
members. 

• Echinoderms are radially symmetric, they have an endoskeleton, some have calcareous spines they 
have a unique water vascular system, a simple digestive system, an open and reduced circulatory system 
and a modified nerve net. 

• Fertilization is generally external; regeneration is fairly common among echinoderms; feeding strategies 
vary greatly. 

• Echinoderms consist of two main subdivisions, the motile Eleutherozoa and the sessile Pelmatazoa. 

• Echinoderms are distributed all over the world at almost all depths, latitudes, and marine environments. 

• Echinoderms play an important role in the ecological community. Economically, they are eaten as deli- 
cacies in different countries, they play a role in cancer research, and they are used as a source of lime. 

Review Questions 

1 . What are the characteristic features of echinoderms? (Intermediate) 

2. What feeding strategies are represented in the echinoderms? (Beginning) 

3. What protection do echinoderms have against predation? (Intermediate) 

4. Chemical elements within the skeleton makes it stronger and more resistant. How could this be an advan- 
tage in grazing echinoderms? (Challenging) 



269 



5. The larvae of many echinoderms, especially starfish and sea urchins, are pelagic (of or pertaining to the 
open ocean). How does this relate to the fact that echinoderms are distributed globally? (Challenging) 

Further Reading I Supplemental Links 

http://dictionary.reference.com 

http://www.oceanicresearch.org/education/wonders/echinoderm.html 

http://www.junglewalk.com/info/echinoderm-information.htm 

http://invertebrates.si.edU/echinoderm/http://en.wikipedia.org 

Vocabulary 

echinodermata The phylum of the echinoderms; contains about 7,000 living species, the largest 

phylum without freshwater or terrestrial members. 

nerve net Interconnected neurons that send signals in all directions. 

pelagic Of, or pertaining to, the open ocean. 

sessile Permanently attached and not freely moving. 

water vascular system A network of fluid-filled canals; functions in gas exchange, feeding, and also in 

locomotion. 

Review Answers 

1 . Echinoderms are radially symmetric; they have an endoskeleton made of tiny calcified plates and spines; 
they have a unique water vascular system used for gas exchange, feeding, and locomotion; they possess 
a simple digestive system; an open and reduced circulatory system; and a modified nerve net. They have 
mostly external fertilization, regeneration and varying feeding strategies. 

2. Some are passive filter feeders; others are grazers; still others are deposit feeders and active hunters. 

3. The endoskeleton made of calcified plates and spines forms a rigid support. Also, some groups, such as 
the sea urchins, have calcareous spines, which protect the organism from predation. 

4. The rock-grazing lifestyle of the sea urchins makes their feeding apparatus (the mandibles) especially 
prone to wear. The extra strength of the skeleton provides it a significant advantage. 

5. The larvae can swim great distances helped by ocean currents and therefore aid in the global distribution 
of the phylum. 

Points to Consider 

• Echinoderms' water vascular system functions in gas exchange via a network of fluid-filled canals. Ter- 
restrial arthropods have internal surfaces that are specialized for gas exchange, via air sacs. How might 
these systems compare and differ? 

• Echinoderms possess an open and reduced circulatory system, consisting of a central ring and five radial 
vessels but no heart. Arthropods also have an open circulatory system but the blood is propelled by a 
series of hearts into the body cavity where it comes in direct contact with the tissues. Why might there 
be an advantage to having a heart as part of the circulatory system? 



270 



Arthropods 

Lesson Objectives 

• Explain what arthropods are. 

• Describe the features of crustaceans. 

• Describe the characteristics of centipedes and millipedes. 

• List the features of arachnids. 

• Describe why arthropods are important. 

Check Your Understanding 

• What is an invertebrate? 

• What do mollusks and echinoderms have in common? 

Introduction 

With over a million described species in the phylum containing arthropods, chances are you encounter one 
of these organisms every day, even without leaving your house. As much as we would like to eliminate all 
insect pests from our dwellings, for example, there is a great probability you will see an ant, a spider, a fly, 
or a moth inside. Even if you don't, you will most likely see such creatures in your yard or on a walk around 
your neighborhood. 

Wherever you observe these animals, you will see a tremendous amount of diversity and adaptations. You 
will also learn, despite how you feel about how annoying some of these organisms may be, how beneficial 
in fact they are both ecologically and economically. 

What are Arthropods ? 

Arthropods belong to the phylum Arthropoda, which means "jointed feet," and includes four living subphyla. 
These are chelicerates, including spiders, mites, scorpions and related organisms; myriapods, comprising 
centipedes and millipedes and their relatives, who are hexapods, including insects and three small orders 
of insect-like animals; and crustaceans, including lobsters, crabs, barnacles, crayfish, and shrimp. 




Figure 1 : A species of spider in its web. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Argi0pe_appensa.jpg, License: GNU-FDL) 



271 





Figure 2: A species of scorpion. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Scorpion.jpg, License: GNU-FDL) 




Figure 3: A centipede, from the subphyla of myriapods. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Centipede.jpg, Photographer: Eric Guinther; License: 
GNU-FDL) 



272 




Figure 4: A species of millipede found in Hawaii. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Millipede.jpg, Photographer: Marshman; License: GNU- 
FDL) 




Figure 5: The blue American lobster illustrates the segmented body plan of the arthropods. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Blue-l0bster.jpg, Photographer: Steven Johnson; License: 
GNU-FDL) 



273 




Figure 6: Giant spider crabs. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Spider_crabs_at_the_Kaiyukan_Aquariurn_in_Osaka 
close.jpg, Photographer: MShades, License: CCA-2.0) 




Figure 7: The sessile barnacles shown here feeding. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Semibalanus_balanoides_upernavik_2007-07-05. jpg, 
Photographer: Kim Hansen, License: GNU-FDL) 



274 




Figure 8: A crayfish. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Blausteinsee_Tierwelt_01 .jpg, License: GNU-FDL) 

Arthropods are characterized by the possession of a segmented body with appendages on at least one 
segment. Arthropod appendages are used for feeding, sensory reception, defense, and locomotion. Their 
heart is on the dorsal side and the nervous system on the ventral. They are covered by a hard exoskeleton 
made of chitin, which provides physical protection and among terrestrial species resistance to drying out. 
In order to grow, arthropods shed this covering in a process called molting. 

It is the largest phylum in the Animal Kingdom with more than a million described species making up more 
than 80% of all described living species. They are found commonly throughout marine, freshwater, terrestrial, 
and even aerial environments, in addition to various forms that are parasitic and symbiotic. They range 
in size from microscopic plankton (approximately % mm) up to the largest living arthropod, the Japanese 
spider crab, with a leg span up to 12 feet (3.5 m). 

Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area in contact with 
the surrounding water. Terrestrial arthropods have internal surfaces that are specialized for gas exchange. 
Insects and most other terrestrial species have a tracheal system, where air sacs lead into the body from 
pores in the exoskeleton, for oxygen exchange. Others use book lungs, or gills modified for breathing air, 
as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen 
absorbing membrane. Terrestrial crabs sometimes have two different structures: one that is gilled, which is 
used for breathing underwater, and another adapted to take up oxygen from the air. 

Arthropods have an open circulatory system with haemolymph, a bloodlike fluid, which is propelled by a 
series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods have a 
complete digestive system with a mouth and anus. 

Crustaceans 

The crustaceans are a large group of arthropods, consisting of almost 52,000 species. The majority of them 
are aquatic, living in either marine or freshwater habitats. A few groups have adapted to living on land, such 
as terrestrial crabs, terrestrial hermit crabs, and woodlice. 



275 




Figure 9: A terrestrial arthropod, a species of woodlice. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:VVoodlice-Scotland.jpg, Photographer: Ross Angus, Li- 
cense: CC-A-2.0) 

Crustaceans are among the most successful animals and are as abundant in the oceans as insects are on 
land. The majority of crustaceans are motile, although a few groups are parasitic and live attached to their 
hosts. Adult barnacles live a sessile life, where they are attached headfirst to the substrate and cannot move 
independently. 

Various parts of the crustacean exoskeleton may be fused together, such as in the carapace, the thick 
dorsal shield seen in many crustaceans that often forms a protective chamber for the gills. The main body 
cavity is an expanded circulatory system, through which blood is pumped by a heart located near the dorsal 
surface. The digestive system consists of a straight tube that often has a gizzard-like gastric mill for 
grinding food and a pair of digestive glands that absorb food. 

Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia 
(connections between nerve cells) close to the antennae and a collection of major ganglia below the gut. 
Most crustaceans have separate sexes. Many terrestrial crustaceans, such as the Christmas Island red 
crab, mate seasonally and return to the sea to release the eggs. Others, such as woodlice, lay their eggs 
on land, although in damp conditions. In other crustaceans, the females keep the eggs until they hatch into 
free-swimming larvae. 

Six classes of crustaceans are generally recognized: 



Class 


Information 


Branchiopoda 


Includes brine shrimp 


Remipedia 


A small class restricted to deep caves connected to salt water 


Cephalo- 
carida 


The horseshoe shrimp 


Maxillopoda 


Includes barnacles and copepods 


Ostracoda 


Small animals with bivalve shells 


Malacostraca 


The largest class, with the largest and most familiar animals: crabs, lobsters, shrimp, krill, 
and woodlice 



Centipedes and Millipedes 

Centipedes and millipedes belong to the subphylum Myriapoda, which contains 1 3,000 species, all of which 
are terrestrial, and which are divided among four classes. They range from having over 750 legs (a species 



276 



of millipede) to having fewer than ten legs. They have a single pair of antennae and simple eyes. 

They are most abundant in moist forests, where they help to break down decaying plant material, although 
a few live in grasslands, semi-arid habitats, or even deserts. The majority are herbivores, but centipedes 
are chiefly nocturnal predators. 

Although not generally considered dangerous to humans, many from this group produce noxious secretions, 
which can cause temporary blistering and discoloration of the skin. Centipedes are fast, predatory, and 
venomous. There are around 3,300 described species, ranging from one tiny species (less than half an inch 
[about 12 mm] in length) to one giant species, which may exceed 12 in (30 cm). 

Most millipedes are slower than centipedes and feed on leaf litter and detritus (loose material, such as 
stone fragments, gravel, or sand, worn away from rocks). Around 8,000 species have been described, although 
there may be as many as 80,000 or more species actually alive. 

The third class, Symphyla, contains 200 species. They resemble centipedes but are smaller and translucent. 
Many spend their lives in the soil, but some live in trees. The pauropods are typically 0.5-2. 0mm long and 
live in the soil of all continents except Antarctica. Over 700 species have been described, and they are be- 
lieved to be closely related to millipedes. 

Arachnids 

Arachnids are a class of joint-legged invertebrates in the subphylum Chelicerata. They are mainly terrestrial, 
but are also found in freshwater and in all marine environments, except for the open ocean. They comprise 
over 100,000 named species, including spiders (Fig. 1), scorpions (Fig. 2), daddy-long-legs, ticks, and mites 
and there may be up to 600,000 species in total, including unknown ones. 




Figure 10: A daddy-long-legs with a captured woodlouse. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Pholcus_phalangioides.jpg, Photographer: Daniel Ullrich, 
Threedots, License: GNU-FDL) 



277 




Figure 11: Various diseases are caused by species of bacteria that are spread to humans by "hard" ticks, 
like the one shown here. 

(Source: http://commons.wikimedia.org/wiki/lmage:Tick_2_(aka).jpg, Photographer: Aka, License: CCA-BY- 
SA-2.5) 




Figure 12: A female crab spider sharing its flower with velvet mites. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Spider_and_mites_May_2008-1.jpg, 
Alvesgaspar, License: GNU-FDL) 



Photographer: 



It is commonly understood that arachnids have four pairs of legs and that they may be easily distinguished 
from insects on this basis (insects have three pairs of legs). Arachnids also have two additional pairs of 
appendages, the first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps, are 
adapted for feeding, locomotion, and/or reproductive functions. Arachnids are further distinguished by the 
fact they have no antennae and no wings. Their body is organized into the cephalothorax, derived from 
the fusion of the head and thorax, and the abdomen. 

Arachnids are also well adapted for a terrestrial existence. They have internal respiratory surfaces in the 
form of trachea or a book lung. They also have appendages modified for more efficient locomotion on land, 
internal fertilization, special sensory organs, and structures for water conservation, such as more efficient 
excretory structures and a waxy layer covering the outer layer of the exoskeleton. 



278 



Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. 
Several groups are largely venomous and they secrete venom from specialized glands to kill prey or enemies. 
Several mites are parasitic and some of those are carriers of disease. Arachnids usually lay eggs, which 
hatch into immature arachnids that resemble the adults. Scorpions, however, bear live young. 

The arachnids are divided into eleven subgroups. Table 2 shows the four most familiar subgroups, with a 
description of each. 



Subgroup 
of Arach- 
nid 


Representa- 
tive Organ- 
isms 


Approxi- 
mate Num- 
ber of 
Species 


Description 


Araneae 


Spiders 


40,000 


Found all over the world, ranging from tropics to the Arctic, some 
in extreme environments; 

All produce silk, used for many functions, including trapping insects 
in webs, aiding in climbing, forming smooth walls for burrows, pro- 
ducing egg sacs, and wrapping prey 

Nearly all spiders inject venom to protect themselves or to kill prey; 
only about 200 species have bites that can be harmful to humans 


Opiliones 


Daddy-long- 
legs 


6,300 


Known for exceptionally long walking legs; no silk nor poison glands 

Many are omnivores, eating small insects, plant material and fungi; 
some are scavengers, eating decaying animal and other matter 

Mostly nocturnal, colored in hues of brown; a number of diurnal 
species have vivid patterns of yellow, green, and black 


Scorpiones 


Scorpions 


2,000 


Characterized by a tail with six segments, the last bearing a pair 
of venom glands and a venom-injecting barb 

Predators of small arthropods and insects, they use pincers to 
catch prey, then either crush it or inject it with a fast-acting venom, 
which is used to kill or paralyze the prey; only a few species are 
harmful to humans 

Nocturnal; during the day find shelter in holes or under rocks 

Unlike the majority of arachnids, scorpions produce live young, 
which are carried about on the mother's back until they have molted 
at least once; they reach an age of between four to 25 years 


Acarina 


Mites and 
ticks 


30,000 


Most are minute to small (no more than 1.0 mm in length), but 
some ticks and one species of mite may reach lengths of 10-20 
mm 

Live in nearly every habitat, including aquatic and terrestrial 

Many are parasitic, affecting both invertebrates and vertebrates, 
and may be vectors of human and other mammalian disease; those 



279 



that feed on plants may damage crops 



Why Arthropods are Important 

Many species of crustaceans, especially the familiar crabs, lobsters, shrimp, prawn, and crayfish, are con- 
sumed by humans, and nearly 10,000,000 tons were produced in 2005. Over 70% by weight of all crustaceans 
caught for consumption are shrimp and prawns, and over 80% is produced in Asia, with China producing 
nearly half the world's total. 

Some mites prey on undesirable arthropods and are used in pest control, while others control weed growth. 
Populations of whip scorpions are valuable in controlling populations of cockroaches and crickets. Finally, 
an unquantified, but major positive contribution of the mites and ticks, as well as the centipedes and millipedes, 
is their role in ecosystems, especially their roles as decomposers and the resulting enriching of the soil due 
to the release of the nutrients during decomposition. 

In the next lesson, we will discuss the diversity of insects. As we will see, insects, also arthropods are ben- 
eficial in many ways, both to the ecosystems of which they are part, as well as to humans. 

Lesson Summary 

• The phylum Arthropoda includes four living subphyla; chelicerates, including spiders, mites, and scorpions; 
myriapods, including centipedes and millipedes; hexapods, including insects; and crustaceans. Arthropods 
are characterized by a segmented body; appendages used for feeding, sensory structures, defense, and 
locomotion; a dorsal heart and a ventral nervous system; and a hard exoskeleton. Arthropods are the 
largest phylum in the Animal Kingdom with more than a million described species; they are found in all 
environments. There are a variety of respiratory systems in arthropods, including gills, tracheal system, 
book lungs, and oxygen absorbing membranes; arthropods have an open circulatory system and a 
complete digestive system. 

• Crustaceans consist of almost 52,000 species, the majority of which are aquatic; they are among the 
most successful animals. There are six classes of crustaceans, including brine shrimp, barnacles and 
copepods, and the malacostracans, including crabs, lobsters, and shrimp. Centipedes and millipedes 
belong to the myriapods, where they occur most abundantly in moist forests; they are chiefly nocturnal 
predators. 

• Arachnids are mainly terrestrial and comprise over 1 00,000 named species; adaptations for a terrestrial 
existence include specialized respiratory structures, appendages modified for locomotion on land, internal 
fertilization, special sensory organs, and structures for water conservation. Arachnids are divided into 
eleven subgroups, the most familiar being spiders; spiders produce silk, which is used in a variety of 
ways. Many species of crustaceans are used for food; some species of mites are used in pest control 
and for controlling weeds; and centipedes, millipedes, and the acarines play a valuable role as decom- 
posers, enriching the soil as a result. 

Review Questions 

1 . What are arthropod appendages used for? (Beginning) 

2. What respiratory systems do terrestrial arthropods use? (Beginning) 

3. Arachnids have several adaptations for living on land. For each adaptation you list, explain how it is 
beneficial for a terrestrial existence. (Challenging) 

4. How does the scorpions' method of producing young differ from most other arachnids? (Intermediate) 
Further Reading I Supplemental Links 

http://cybersleuth-kids.com/sleuth/Science/Animals/Arthropods/index.htm 



280 



http://www.oceanicresearch.org/education/wonders/arthropods.htm 
http://www.biokids.umich.edu/critters/Crustacea 
http://www.nps.gOv/archive/yell/kidstuff/Alphabet/a.htm 
Vocabulary 



acanna 
araneae 
arthropoda 
book lungs 
carapace 

cephalothorax 

chelicerae 

chelicerata 

ganglia 

gastric mill 

haemolymph 

molting 

myriapoda 

opiliones 

parasitic 

pedipalps 

scorpiones 
silk 

symbiotic 



The group of arachnids containing the mites and ticks. 

The arachnid group containing the spiders. 

The phylum meaning "jointed feet;" includes four living subphyla of arthropods. 

Gills modified for breathing air. 

The thick dorsal shield seen in many crustaceans; often forms a protective chamber for 
the gills. 

The anterior part of the arachnid body, derived from the fusion of the head and thorax. 

The first pair of arachnid appendages; used in feeding and defense. 

An arthropod subphylum containing the arachnids. 

A compact group of nerve cells having a specific function. 

A gizzard-like structure for grinding food. 

A bloodlike fluid, which is propelled by a series of hearts into the body cavity, where it 
comes in direct contact with the tissues. 

The process by which arthropods shed their hard exoskeleton in order to grow. 

An arthropod subphylum containing the centipedes and millipedes. 

The arachnid group containing daddy-long-legs. 

Living on or in an organism of another species; harmful to the host species. 

The second pair of arachnid appendages used for feeding, locomotion, and/or reproductive 
functions. 

The group of arachnids containing the scorpions. 

A thin, strong, protein strand extruded from the spinnerets; most commonly found on the 
end of the abdomen of spiders. 

The living together of two dissimilar organisms. 



Review Answers 

1. They are used for feeding, sensory reception, defense, and locomotion. 

2. They have internal surfaces that are specialized for gas exchange. Insects and most other terrestrial 
species have a tracheal system. Others use book lungs, or gills modified for breathing air. Some areas of 
the legs of soldier crabs are covered with an oxygen absorbing membrane. Terrestrial crabs sometimes 
have two different structures, one is used for breathing underwater and the other is adapted to take up 
oxygen from the air. 

3. Internal respiratory surfaces in the form of a trachea or book lung are necessary for absorbing oxygen 
directly from the air. Appendages are modified for more efficient locomotion on land. Special sensory organs 
are adapted for being able to move about on land and for detecting prey and predators on land. Water must 
be conserved on land and so there are more efficient excretory structures (less excretion of liquids) and a 
waxy layer covering the outer layer of the exoskeleton to minimize water loss. 

4. Arachnids usually lay eggs, which hatch into immature arachnids that resemble the adults. Scorpions are 
live bearing. The young are carried about on the mother's back until the young have undergone at least one 



281 



molt. The young generally look like their parents and require between five and seven molts to reach maturity. 

Points to Consider 

• Arthropods are characterized by the possession of a segmented body with appendages on at least one 
segment and they are covered by a hard exoskeleton made of chitin. How is the general arthropod body 
plan specialized in the insects? 

• Insects are the only group of invertebrates to have developed flight. Compare this mode of locomotion 
to those discussed in the groups of arthropods already discussed. What advantages might there be to 
using flight for a method of locomotion? 



Insects 

Lesson Objectives 

• Describe the characteristics of insects. 

• Explain how insects obtain food. 

• Describe reproduction and the life cycle of insects. 

• Explain how insects are important. 

• Describe how insect pests are controlled. 

Check Your Understanding 

• What is an arthropod? 

• Is a spider an insect? Why or why not? 

Introduction 

Insects, with over a million described species, are the most diverse group of animals on Earth. They may 
be found in nearly all environments on the planet. That would explain that no matter where you travel, you 
are bound to see representatives from this group and probably lots of different kinds as well. Even if you 
were not partial to bees, wasps, and ants perhaps, it would be difficult to not admire the beauty of a butterfly, 
moth, or even a dragonfly! 

As you learn about the amazing diversity within this group and some of the fascinating behaviors, you may 
begin to look upon some of the insects you come upon with a bit more interest! Perhaps you will even learn 
to appreciate some of the species you may dislike now, such as bees and wasps, when you realize how 
beneficial they are to humans and especially necessary for the continued presence of some of the beautiful 
flowers or delicious fruits that may grace your yard or nearby park. 

What Are Insects? 

Insects are a major group of arthropods and the most diverse group of animals on the planet, with over a 
million described species and more than half of all known living organisms. They are found in nearly all en- 
vironments on Earth, although only a few species occur in the oceans. Adults range in size from a minuscule 
fairy fly to a 21 .9 in (55.5 cm) long stick insect. 



282 



it* 

a ■-.■;. 




Figure 1 : A stick insect, showing how well it blends in to its environment. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Ctenomorpha_chronus.jpg, License: GNU-FDL) 

Insects have segmented bodies with an exoskeleton. The outer layer of the exoskeleton, the cuticle, is 
made up of two layers, a thin and waxy water resistant outer layer (the exocuticle), and an inner, much 
thicker layer. The exocuticle is greatly reduced in many soft-bodied insects and especially in larval stages, 
such as caterpillars. 




Figure 2: Caterpillars feeding on a host plant. 

(Source: http://commons.wikimedia.org/wiki/lmage:Danaus_plexippus- caterpillars.jpg, Photographer: Engeser, 
License: GNU-FDL) 

The segments of the body are organized into three distinctive but joined units: a head, a thorax, and an 
abdomen. 



283 




Figure 3: A diagram of a human and an insect, comparing the three main body parts: head, thorax, and 
abdomen. 

(Source: http://commons.wikimedia.org/wiki/lmage:Thorax_(PSF).png Image: Pearson Scott Foresman, Li- 
cense: Public Domain) 

Table 1 shows the structures present in each body segment. 



Head 


Thorax 


Abdomen 


A pair of sensory antennae, a pair of com- 
pound eyes, one to three simple eyes, and 
three sets of variously modified appendages 
that form the mouthparts 


Six segmented legs and 
two or four wings 


Has most of the digestive, respira- 
tory, excretory, and reproductive 
structures 



Table 1: Insect Structures 

The nervous system is divided into a brain and a ventral nerve cord. Air is taken in through the spiracles, 
openings on the sides of the abdomen. Insect respiration occurs without lungs, with a system of internal 
tubes and sacs through which oxygen is delivered directly to the adjoining body tissues. Since oxygen is 
delivered directly, the circulatory system is therefore greatly reduced and consists of only a single dorsal 
tube with openings. The tube pulses and circulates blood-like fluids inside the body cavity. 

Insect locomotion includes flight, walking, and swimming. Insects are the only invertebrates to have developed 
flight and this has played an important role in their success. Insect flight is not very well understood. Primitive 
insect groups use muscles that act directly on the wing structure. More advanced groups have foldable 
wings and their muscles act on the wall of the thorax and give power to the wings indirectly. These muscles 
are able to contract multiple times for each single nerve impulse, allowing the wings to beat faster than would 
ordinarily happen. 

Many adult insects use six legs for walking and have adopted a gait that uses the legs in alternate triangles 
touching the ground. This gait allows for rapid walking at the same time as having a stable stance. A few 
insects have evolved to walk on the surface of the water, especially the water striders. 



284 




Figure 4: A pair of water striders mating, showing how water surface tension allows for them to stand on 
the water. 

(Source: http://commons.wikimedia.0rg/wiki/Bild:VVasserlaufer_bei_der_Paarung_crop.jpg, License: GNU- 
FDL) 

A large number of insects live either parts of or their whole lives underwater. Water beetles and water bugs 
have legs adapted to paddle in the water. Dragonfly young use jet propulsion, forcibly expelling water out 
of the rectal chamber. 

Insects use a wide variety of senses for both communicating and receiving information. Many insects have 
very sensitive and/or specialized sensory organs. Table 2 summarizes five types of communication that are 
used by various insects and sometimes for different purposes. 



Types of Communication 


Representative 
Organisms 


Description 


Visual 


Bees 


Perceive ultraviolet wavelengths 


Ultraviolet wavelengths 


Bees Fireflies 


Detect polarized light 


Polarized light 




Reproduction and Predation 


Bioluminescence 




Some species produce flashes to attract mates; 
other species to lure prey. 


Sound Production 


Cicadas 


Loudest sounds among insects; have special 
modifications of body and musculature to produce 


Mostly by mechanical action of 


Moths 


and amplify sounds. 


appendages 








Moths 


Predation 


Ultrasound clicks 








Some predatory 


Produced mostly by unpalatable moths to warn 


Hearing 


and parasitic in- 


bats; other moths make similar sounds in order to 




sects 


mimic distasteful moths so they will be avoided by 
bats as well. 

Predation 



285 







Some nocturnal species can hear the ultrasonic 
emissions of bats, which help them avoid predation. 

Can detect sounds made by prey or hosts. 


Chemical 

Wide range of insects have 
evolved chemical communication; 
chemicals often derived from plant 
metabolites and are used to attract, 
repel, or provide other kinds of infor- 
mation; chemicals may be targeted 
at individuals of same or different 
species; use of scents is especially 
well developed in social insects. 


Moths 


Antennae of males can detect pheromones 
(chemicals secreted by animals, especially insects, 
that influence the behavior or development of others 
within the same species) of female moths over dis- 
tances of many kilometers. 


Infrared 


Blood-sucking in- 
sects 


Have specialized sensory structures that can detect 
infrared emissions in order to find their hosts. 


"Dance Language" - a system of 
abstract symbolic communication 


Honey bees 


Thought that various species of honey bees are 
only invertebrates to have evolved this type of 
communication; angle at which bee dances repre- 
sents direction relative to sun, length of dance rep- 
resents distance to be flown. 



Table 2: Insect Communication 




Figure 5: A yellow-collared scape moth, showing the feathery antennae. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Yellow-collared_Scape_Moth_2.jpg, Photographer: Benny 
Mazur, License: CCA-2.0) 

Social insects, such as the termites, ants, and many bees and wasps, are the most familiar social species. 
They live together in large well-organized colonies. Only those insects which live in nests or colonies show 
any true capacity for homing. This allows an insect to return to a single hole among a mass of thousands 
of apparently identical holes, after a trip of up to several kilometers and as long as a year after last seeing 
the area, as when an insect hibernates. A few insects migrate, but this is a larger-scale form of navigation 
and involves only a large general region, such as the overwintering of the monarch butterfly. 



286 




Figure 6: Damage to this nest, brings the workers and soldiers of this social insect, the termite, to repair it. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Nest_0f_F0rm0san_subterranean_termites.jpg, Photog- 
rapher: Scott Bauer, License: Public Domain - ARS) 




Figure 7: A wasp building its nest. 

(Source: http://commons.wi kimedia.org/wiki/lmage:Wasp_building_nest_in_Barcelona_(flo).jpg, Pftofograp/jer; 
Florian Siebeck, License: CC-BY-SA-2.0 Germany) 



287 





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Figure 8: Monarch butterflies in an overwintering cluster. 

(Source: http://comrnons.wikimedia.Org/wiki/lmage:Monarch_butterflies_cluster_in_SC_3.jpg, Photographer: 
Mila Zinkova, License: GNU-FDL) 

Insects are divided into two major groups, the wingless and the winged insects. The wingless consists of 
two orders: the bristle tails and the silverfish. The winged orders of insects include the mayflies; dragonflies 
and damselflies; stoneflies; webspinners; angel insects; earwigs; grasshoppers, crickets, and katydids; stick 
insects; ice-crawlers and gladiators; cockroaches and termites; mantids; lice; thrips; true bugs, aphids, and 
cicadas; wasps, bees, and ants; beetles; twisted-winged parasites; snakeflies; alderflies and dobsonflies; 
lacewings and antlions; scorpionflies and hangingflies (including fleas); true flies; caddisflies; and butterflies, 
moths, and skippers. 

How Insects Obtain Food 

Insects have a wide variety of appendages adapted for capturing and feeding on prey. In addition, as already 
discussed, they have sensory capabilities, which help them detect prey. 

Insects have a wide range of mouthparts used for feeding. Specialized parts are mostly for piercing and 
sucking, as in mosquitoes and aphids. A number of insect orders have mouthparts that pierce food items 
to enable sucking of internal fluids. Some are herbivorous, like aphids and leafhoppers, while others are 
insectivorous, like assassin bugs and mosquitoes (females only). 

Examples of chewing insects include dragonflies, grasshoppers, and beetles. Some larvae have chewing 
mouthparts, as in moths and butterflies. 

Some insects use siphoning, as if sucking through a straw, as in moths and butterflies, where some of the 
mouthparts are adapted into an elongated sucking tube. You have probably seen a butterfly or moth poised 
at a flower while it siphons the nectar of the flower. Some moths, however, have no mouthparts at all. 

Some insects are capable of sponging, as in the housefly. One of the mouthparts is specialized for this 
function, where liquid food is channeled to the esophagus. The housefly is able to eat solid food by secreting 
saliva and dabbing it over the food item. As the saliva dissolves the food, the sponging mouthpart absorbs 
the liquid food. 

Reproduction and Life Cycle of Insects 

Most insects have a high reproductive rate and can rapidly reproduce within a short period of time. With a 
short generation time, they evolve faster and can adjust to environmental changes faster. Although there 
are many forms of reproductive organs in insects, there is a basic design and function for each reproductive 



288 



part. These parts may vary in shape (gonads), position, and number (glands), with different insect groups. 

Most insects reproduce via sexual reproduction. The female produces eggs, which are fertilized by the male, 
and then the eggs are usually deposited in a precise microhabitat at or near the required food. Most insects 
are oviviparous, where the young hatch after the eggs have been laid. In some insects, there is asexual 
reproduction and in the most common type, the offspring are essentially identical to the mother. This is most 
often seen in aphids and scale insects. 

An insect can have one of three types of metamorphosis and life cycle: 



Type of Meta- 
morphosis 


None 


Incomplete 


Complete 


Characteris- 
tics 


Only difference 
between adult 
and larvae is 
size 


Young, called nymphs, usu- 
ally similar to adult, wings 
then appear as buds on 
nymphs or early forms; when 
last molt is completed wings 
expand to full adult size 


Insects have different forms in immature and 
adult stages, have different behaviors, and 
live in different habitats; immature form is 
called larvae and remains similar in form but 
increases in size; they usually have chewing 
mouthparts even if adult mouthparts are 
sucking ones; at last larval stage of develop- 
ment insect forms into pupa, doesn't feed and 
is inactive; here wing development is initiated, 
and adult emerges 


Example 


Silverfish 


Dragonflies 


Butterflies and Moths 



[^HBBHHHHHSi 




Figure 9: Heteroptera nymphs and egg cases. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Heter0ptera_nymphs.JPG, Photographer: Bohringer 
friedrich, License: CC-BY-SA-2.5) 



289 




Figure 10: The chrysalis (pupal stage) of a monarch butterfly. 

(Source: http://cornrnons.wikimedia.Org/wiki/lmage:Monarch_Butterfly_Chrysalis.jpg, License: GNU-FDL) 

Importance of Insects 

Many insects are considered to be pests by humans. In spite of this, insects are also very important. In the 
environment, some insects pollinate flowering plants, as in wasps, bees, butterflies, and ants. Many insects, 
especially beetles, are scavengers, feeding on dead animals and fallen trees, and insects are responsible 
for much of the process by which topsoil is created. 

Insects also produce useful substances as honey, wax, lacquer, and silk. Honeybees have been cultured 
by humans for thousands of years for honey. The silkworm has greatly affected human history, as silk-driven 
trade established relationships between China and the rest of the world. 

Fly larvae (maggots) were formerly used to treat wounds to prevent or stop gangrene, as they would only 
consume dead flesh. This treatment is finding modern usage in some hospitals. Adult insects such as 
crickets, and insect larvae of various kinds, are also commonly used as fishing bait. 

In some parts of the world, insects are used for human food, while being a taboo in other places. Some 
people support this idea to provide a source of protein in human nutrition. Insects also have a role in controlling 
insect pests, as we will see in the next section. 

Controlling Insect Pests 

Insects commonly regarded as pests include those that are parasitic (mosquitoes, lice, bed bugs), transmit 
diseases (mosquitoes, flies), damage structures (termites), or destroy agricultural products (locusts, weevils). 
Many entomologists are involved in various forms of pest control, often using insecticides, but more and 
more relying on methods of biocontrol. 

Biological control of pests in agriculture is a method of controlling pests that relies on predation, parasitism, 
herbivory, or other natural mechanisms. Insect predators, such as lady beetles and lacewings, are mainly 
free-living species that consume a large number of prey during their lifetime. 

Parasitoids are species whose immature stage develops on or within a single insect host, ultimately killing 
the host. Most have a very narrow host range. Many species of wasps and some flies are parasitoids. Both 



290 



of these types of predators and parasitoids are used to control insect pests. Pathogens are disease-causing 
organisms including bacteria, fungi, and viruses, which kill or debilitate their host and are specific to certain 
insect groups. 

Most of the insecticides now applied are long-lasting synthetic compounds that affect the nervous system 
of insects on contact. Agricultural pesticides prevent a monetary loss of about $9 billion each year in the 
U.S. These benefits, however, must be weighed against the costs to society of using pesticides, which include 
human poisonings, fish kills, honeybee poisonings, and the contamination of livestock products. 

Lesson Summary 

• Insects are the most diverse group of animals on Earth; they are found in nearly all environments. They 
have segmented bodies with an exoskeleton; the nervous, respiratory, and circulatory systems are fairly 
simple. Insects are the only invertebrates to have developed flight. Insects have very sensitive and/or 
specialized organs of perception, including visual, chemical, heat-sensitive, and auditory. Some insects, 
like termites, ants, and many bees and wasps, are social and live together in large well-organized colonies. 

• Insect locomotion includes flight, walking, and swimming. There are two major groups of insects, the 
wingless and the winged, and these are further subdivided into various orders. Insects obtain food with 
the use of specialized appendages for capturing and eating the prey. Most insects have a high reproductive 
rate and can rapidly reproduce within a short period of time. An insect can have one of three types of 
metamorphosis and life cycle. Insects are beneficial both environmentally and economically. Insect pests 
can be controlled with chemical or with natural means, some of which are insects themselves; even 
though agricultural pesticides prevent a major monetary loss, they have major drawbacks, too. 

Review Questions 

1 . What are the main characteristics of insects? (Beginning) 

2. Why is the insect's circulatory system greatly reduced? (Intermediate) 

3. Give an example of mimicry in insects. (Challenging) 

4. How do female accessory glands aid in the development of eggs? (Intermediate) 

5. What makes parasitoids especially effective against pests? (Intermediate) 
Further Reading I Supplemental Links 
http://homeschooling.gomilpitas.com/explore/bugs.htm 
http://rusinsects.com/links/view.php?id=20 
http://www.kidsolr.com/science/page18.html 
http://pestworldforkids.org/learninggames.html 

Vocabulary 

cuticle The outer layer of the exoskeleton. 

exocuticle The thin and waxy water resistant outer layer of the cuticle. 

nymphs A developmental stage of insects, where the young is usually similar to the adult. 

oviviparous A method of reproduction where the young hatch after the eggs have been laid. 

parasitoids Species whose immature stages develop on or within a single insect host, ultimately killing 
the host. 

pheromones Chemicals secreted by animals, especially insects, that influence the behavior or development 
of others within the same species. 



291 



spiracles Openings on the sides of the insect abdomen, through which air is taken in. 

Review Answers 

1. They have segmented bodies with an exoskeleton. The segments of the body are organized into three 
distinctive but joined units. The nervous system is divided into a brain and a ventral nerve cord. Respiration 
occurs without lungs. 

2. Oxygen is directly delivered to the body tissues. 

3. Some moths produce ultrasound clicks that are similar to those of moths that are distasteful to bats. These 
moths get avoided by bats, as well. 

4. Accessory glands secrete an adhesive substance for attaching eggs to an object and supplies material 
for a protective coating for the egg. 

5. Parasitoids develop on or within a single insect host. 

Points to Consider 

• Some of the adaptations that insects have evolved for a terrestrial existence are also displayed in am- 
phibians and reptiles. What could be some of these? How are they similar and different? 

• Insects have some very specialized sensory capabilities. How do you think these compare to those found 
in fish, amphibians, and reptiles? 



292 



13. Fishes, Amphibians, and Reptiles 



Introduction to Vertebrates 

Lesson Objectives 

• Describe the general features of chordates. 

• List the three groups of chordates with their characteristics. 

• List the general features of vertebrates. 

• Describe the classification of vertebrates. 

Check Your Understanding 

1 . What is the function of the notochord in lower vertebrates? 

2. What happens to the notochord in higher vertebrates? 

introduction 

It is hard to believe that some of the organisms that are chordates are closely related to us and vertebrates 
like us - everything from fish to amphibians and reptiles, to birds and mammals. Chordates are a group of 
animals that includes the vertebrates, as well as several closely related invertebrates. Some chordates, as 
we will soon see, appear to be nothing more than animals resembling marine invertebrates, like the tunicates in 
Figure 1 . Chordates also include the lancelets, which appear as mostly featureless and simplified swimming 
animals (Figure 2). What these all have in common, though, are certain characteristics appearing either in 
the larval or adult forms, and which we will explore further in the first section. 

Vertebrates all have backbones or spinal columns as well as some other defining characteristics. About 
58,000 species have been described and contain many familiar groups of large land animals. 

Chordates 

Chordates (phylum Chordata), including the vertebrates and several closely related invertebrates, are united 
by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord); pharyngeal slits (vertical 
slits in the pharynx wall, which help to filter out food particles); an endostyle (ciliated groove or grooves lo- 
cated in the pharynx), and a post-anal tail. The phylum is broken down into three subphyla: Urochordata 
(represented by tunicates), Cephalochordata (represented by lancelets) and Vertebrata (the vertebrates). 

Urochordates have a notochord and nerve cord only during the larval stage and cephalochordates have 
a notochord and nerve cord but no vertebrae (bones in the backbone). In all vertebrates, except for hagfish, 
the notochord is generally reduced and the dorsal hollow nerve cord is surrounded with cartilaginous (made 
of cartilage, not bone) or bony vertebrae. 

The urochordates consist of 3,000 species of tunicates (sessile (permanently attached) marine animals, 
with saclike bodies having thick membranes and siphons for water movement) and the cephalochordates 
consist of 30 species of lancelets (burrowing marine animals). The vertebrates encompass 57,739 species, 
including jawless and jawed vertebrates. 

The origin of chordates is currently unknown. The first clearly identifiable chordates appear in the Cambrian 
Period (about 542 - 488 million years ago) as lancelet-like specimens. 



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Figure 1: Tunicate colonies of Botrylloides violaceus (subphylum urochordata), showing oral tentacles at 
openings of oral siphons, which take in food and water, and expel waste and water. 

(Source: http://commons.wikimedia.0rg/lmage:Botrylloides_violaceus.jpg, License: Public Domain) 




Figure 2: Pikaia gracilens (subphylum cephalochordates), perhaps the oldest known ancestor of modern 
vertebrates, resembled a living chordate, known as a lancelet, and perhaps swam much like an eel. Pikaia 
is thought to have had a very primitive, proto-notochord. Its "tentacles" may be related to those in present- 
day hagfish, ajawless chordate. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Pikaia_BW.jpg, License: GNU-FDL) 

What are Vertebrates? 

Vertebrates, belonging to the subphylum Vertebrata, are chordates with a backbone or spinal column. Other 
characteristics are a braincase, or cranium, and an internal skeleton (the latter feature is present in all 
vertebrates except for lampreys). All vertebrates are most easily distinguished from all other chordates by 
having a defined head with pronounced cephalization. Cephalization is the concentration of nervous tissue 
towards one end of the organism. Vertebrates have sensory organs, especially eyes, concentrated at the 
front (anterior) end of the body. How do you think this type of body design is an advantage? 

Typical vertebrate traits include: 

• a backbone or spinal column 

• braincase 

• internal skeleton 

• defined head with pronounced cephalization 



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• sensory organs, especially eyes 

The vertebrate muscular system mostly consists of paired masses, as well as a central nervous system, 
partly located inside the backbone, when a backbone is present. Extant (living) vertebrates range in size 
from a carp species (Figure 3), at as little as 7.9 mm (0.3 in), to the blue whale, as large as 110 ft (Figure 
4). 




Figure 3: A species of carp, carp bream (Abramis brama). 

(Source: http://commons.wikimedia.org/wiki/lmage:Braxen,_lduns_kokbok.jpg, License: Public Domain) 




Figure 4: An image of the blue whale, the largest living vertebrate, reaching up to 33 m (110 ft) long. Shown 
below it is the smallest whale species, Hector's dolphin (about 1 .4 m (5 ft) in length), and beside it, a human. 

(Source: http://commons.wikimedia.Org/wiki/lmage:lmage-Blue_Whale_and_Hector_Dolphine_Colored.jpg, 
License: Public Domain) 

Classification of Vertebrates 

Vertebrates consist of both jawless and jawed vertebrates. The jawless vertebrates consist of more than 
100 species including 65 species of hagfish, the conodonts, and the lampreys. The jawed vertebrates include 
over 900 species of cartilaginous fish , over 30,000 species of bony fish and over 1 8,000 species of tetrapods, 
or four-legged (or leg-like) vertebrates. 

The bony fish are further divided into ray-finned and lobe-finned fish. The tetrapods consist of amphibians, 
reptiles, birds, mammal-like reptiles, and mammals . 

Table 1: Species of the Main Groups of Tetrapods 



Type of Tetrapod 


Number of Species 


Amphibians 


6,000 


Reptiles 


8,225 


Birds 


10,000 


Mammal-like Reptiles 


4,500 



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Mammals 5,800 



Lesson Summary 

• Chordates are characterized by a notochord, pharyngeal slits, an endostyle, and a post-anal tail. 

• There are three main groups of chordates, including tunicates, lancelets and vertebrates. 

• Vertebrates are distinguished by having a backbone or spinal column. 

• Vertebrates are classified into two major groups: those without jaws and those with jaws. 

Review Questions 

1 . What features characterize the chordates? (Beginning) 

2. What are the main features of vertebrates? (Beginning) 

3. The first clearly-identifiable chordates are lancelet-like (small, burrowing marine animals with a lancet 
shape) specimens. List three ways in which these first chordates could have evolved into a swimming-like 
animal. (Challenging) 

4. Which two structures that all chordates possess sometime during their life cycle are used for food gathering, 
and how are these structures used? (Challenging) 

5. Why, do you think, cephalization is not necessary in urochordates and cephalochordates? Explain how 
this is illustrated in tunicates. (Challenging) 

Further Reading I Supplemental Links 

http://www.ucmp.berkeley.edu/chordata/Chordata.html 

http://www.ucmp.berkeley.edu/vertebrates/vertintro.html 

http://en.wikipedia.org/wiki 

Vocabulary 

cephalization The placement of important sensory organs near or in the head. 

cephalochordates A group of chordates with a notochord and nerve cord but no vertebrae. 

chordata The phylum of chordates, containing the vertebrates and several closely related inver- 

tebrates. 
cranium The braincase or skull. 

endostyle A groove or pair of grooves having cilia; located in the pharynx; functions are to gather 

food particles and transport them along the digestive tract. 

notochord A hollow dorsal nerve cord. 

urochordates A group of chordates having a notochord and nerve cord present only during the larval 

stage. 

vertebrata The subphylum of vertebrates, distinguished by having backbones or spinal columns. 

Review Answers 

1. Chordates have a notochord, pharyngeal slits, an endostyle, and a post-anal tail. 



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1. Vertebrates have a backbone or spinal column, a cranium, an internal skeleton, a defined head with 
sensory organs, paired muscles and a central nervous system. 

1. A sediment-dwelling worm-like animal could have evolved a flatter body and/or fins for swimming. A 
permanently attached (sessile) tubular filter-feeder (such as a tunicate) has a tadpole-like larva. Such an 
organism could have evolved into a free-swimming animal via usage of fins. A drifting or swimming larva 
of some other kind of animal that eventually retained its swimming features into adulthood. 

1. One structure is the pharyngeal slits, which help to filter out food particles. The other is the endostyle, 
which has cilia, used to gather food particles and transport them along the digestive tract. 

1. Because of their lack of locomotion and because of their feeding strategy (tunicates are sessile, and wait 
for prey items to come to them). 

Points to Consider 

• The notochord's stiffness in many chordates may have evolved to facilitate the effectiveness of swimming 
in S-shaped movements. Think about the advantages this may have for water-living vertebrates. 

• Unlike chordates with cephalization, cephalochordates (lancelets)have a mouth, but not a well-developed 
head, and have light-sensitive areas along their entire back, instead of at the anterior end of the body. 

• How do you think cephalization could be an advantage in movement and feeding in fish? 



Fishes 

Lesson Objectives 

List the general traits offish. 

Describe the features of jawless fish. 

List the general features of the cartilaginous fish. 

Describe the features of bony fish and the significance of this superclass. 

List some of the reasons why fish are important. 

Check Your Understanding 

1 . What are the unique characteristics of vertebrates? 

2. What are the two main groups of vertebrates? 

Introduction 

So what exactly is a fish? You probably think the answer is obvious. You may say that a fish is an animal 
that swims in the ocean or a lake. But there is lots more to fish than that. Fish are aquatic vertebrates, which 
through evolution became a dominant form of sea life and eventually branched to create land vertebrates. 
They have a number of characteristic traits and are classified into two major groups: jawless and jawed 
fish. Jawed fish are further divided into those with bones and those with just cartilage. Fish, in general, are 
important in many ways to humans - economically, recreationally and culturally. Perhaps you can think of 
some of these ways? 

Characteristics of Fish 

Fish are vertebrates that are typically ectothermic, are covered with scales, have jaws and have two sets 
of paired fins and several unpaired fins. A typical fish has a streamlined body that allows it to swim rapidly, 



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extracts oxygen from the water using gills or an accessory breathing organ to enable it to breathe atmospheric 
oxygen, and lays eggs that are fertilized internally or externally. Fish range in size from the 16 m (51 ft) 
whale shark to the 8 mm (just over % of an inch) stout infantfish. 

Traits of a typical fish include: 

Vertebrate 

Ectothermic 

Scales 

Jaws 

Two sets of paired fins 

Several unpaired fins 

Streamlined body 

Gills or an accessory breathing organ 

Lays eggs that are fertilized internally or externally 




Figure 1: The humphead or Napoleon wrasse (Cheilinus undulates), showing some of the general traits of 
fish, including scales, fins and a streamlined body. 

(Source: http://commons.wikimedia.Org/lmage:Cheilinus_undulatus_1.jpg, License: GNU-FDL) 

There are exceptions to many of these traits. For example, tuna, swordfish, and some species of sharks 
show some warm-blooded adaptations, and are able to raise their body temperature significantly above that 
of the water around them. Some species offish have a slower, but more maneuverable, swimming style, 
like eels and rays (Figure 2). Body shape and the arrangement of fins are highly variable, and the surface 
of the skin may be naked, as in moray eels, or covered with scales. Scales can be of a variety of different 
types. 



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Figure 2: One of the cartilaginous fish, a stingray, showing very flexible pectoral fins connected to the head. 

(Source: http://c0mm0ns.wikimedia.0rg/lmage:Stingray_underwater.jpg, L/'cense.Creative Commons Attri- 
bution 2.0) 

Although most fish live in aquatic habitats, such as the ocean, lakes, and rivers, there are some that spend 
considerable time out of water. Mudskippers, for example, feed and interact with each other on mudflats for 
up to several days at a time and only go underwater when occupying burrows (Figure 3). They breathe by 
absorbing oxygen across the skin, similar to what frogs do. 




Figure 3: A mudskipper, shown on the mudflats, where it spends time feeding and interacting with other 
individuals. 

(Source: http://commons.wikimedia.0rg/lmage:Periophthalmus_modestus.jpg, License: CCA-BY-SA 2.5) 

Agnatha: Jawless Fishes 

Agnatha is a superclass of jawless fish belonging to the phylum Chordata, subphylum Vertebrata (agnath 
means jawless). There are two extant (living) groups of jawless fish, the lampreys and the hagfish, with 
about 100 species in total. Although hagfish belong to the subphylum Vertebrata, they do not technically 
have vertebrae. 

In addition to the absence of jaws, Agnatha are characterized by absence of paired fins, the presence of a 
notochord both in larvae and adults, and seven or more paired gill pouches. The branchial arches (a series 
of arches that support the gills of aquatic amphibians and fishes) lie close to the body surface. 

Agnatha have a light sensitive pineal eye (an eyelike structure that develops in some cold-blooded vertebrates) 
and do not have an identifiable stomach. They reproduce using external fertilization. They are ectother- 



299 



mic, have a cartilaginous skeleton, and a heart with two chambers. 

Many agnathans from the fossil record were armored with heavy bony-spiky plates. The first armored ag- 
nathans - the Ostracoderms - were precursors to the bony fish and hence to the tetrapods, including humans. 

What advantages would the advent of jaws have for fish? Such an adaptation would allow fish to eat a much 
wider variety of food, including plants and other organisms. In the next two sections you will be introduced 
to two groups offish with jaws: those with a cartilaginous skeleton and those with a bony skeleton. 

Cartilaginous Fishes 

The cartilaginous fishes, or Chondrichthyes, are jawed fish with paired fins, paired nostrils, scales, two- 
chambered hearts, and skeletons made of cartilage rather than bone. The approximate 1,000 species are 
subdivided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaera, 
sometimes called ghost sharks). Fish from this group range in size from the dwarf lanternshark, at 16 cm 
(6.3 in), to the whale shark, up to sizes of 13.6 m (45 ft) (Figure 4). 

Figure 4: One of two male whale sharks at the Georgia Aquarium. Whale sharks are the largest cartilaginous 
fish. 

(Source: http://commons.wikimedia.Org/lmage:Male_whale_shark_at_Georgia_Aquarium.jpg, Photographer: 
Zac Wolf, License: CCA-BY-SA 2.5) 

Animals from this group generally have ratio of brain weight to body size that is close to that of mammals, 
and about ten times that of bony fishes. One of the explanations for their relatively large brains is that the 
density of nerve cells is much lower than in the brains of bony fishes, making the brain less energy demanding 
and allowing it to be bigger. 

Since they do not have bone marrow (as they have no bones), red blood cells are produced in the spleen, 
in special tissue around the gonads, and in an organ called Leydig's Organ, only found in cartilaginous 
fishes. The tough skin of this group is covered with dermal teeth, or placoid scales, although they are mostly 
lost in adult Holocephali, making it feel like sandpaper. It is assumed that their oral teeth evolved from these 
dermal teeth, which migrated into the mouth. 

The sharks, rays and skates are further broken into two superorders, one containing the rays and skates, 
and the other containing the sharks. There are eight orders of sharks within the superorder. They are distin- 
guished by such features as: 

Number of gill slits 

Numbers and types of fins 

Type of teeth 

Body shape 

The sawsharks, with an elongate, toothed snout used for slashing the fish that they eat. 

The bullhead sharks, with teeth used for grasping and crushing shellfish. 

Carpet sharks with barbels 

Nocturnal habits 

The groundsharks, with an elongated snout. 

The mackerel sharks, with large jaws and ovoviviparous reproduction, where the eggs develop inside 
the mother's body after internal fertilization, and the young are born alive. 



300 




Figure 5: A spotted Wobbegong shark (Orectolobus maculatus), at Shelly Beach, Sydney, Australia, 
showing skin flaps around the mouth and cryptic coloration. 

(Source: http://commons.wikimedia.org/lmageWobbegong.jpg, License: GNU-FDL) 

Bony Fishes 

The Osteichthyes, or bony fish, contain almost 27,000 species, which are divided into two classes: the ray- 
finned fish (Actinopterygii) and the lobe finned fish (Sarcopterygii). Most bony-fish belong to the 
Actinopterygii; there are only eight living species of lobe finned fish, including the lungfish and coelacanths. 




Figure 6: One of the only eight living species of lobe finned fish, the lungfish. 

(Source: http://commons.wikimedia.Org/lmage:VancouverZooEel.jpg, Photographer: Chris Stubbs, License: 
GNU-FDL) 



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Figure 7: One of the eight living species of lobe finned fish, the coelacanth. 

(Source: http://c0mm0ns.wikimedia.0rg/lmage:Latimeria_menad0ensis.jpg, License: CCA-BY-SA 2.5) 

The vast majority offish are osteichthyes, and this group is the most various of vertebrates, making them 
the largest group of vertebrates in existence today. They are characterized by a relatively stable pattern of 
cranial bones, and the head and pectoral girdles (arches supporting the forelimbs) are covered with large 
dermal bones (bones derived from the skin). They have a lung or swim bladder, which helps the body create 
a neutral balance between sinking and floating, by either filling up with or emitting such gases as oxygen; 
have bone fin rays (jointed, segmented rods) supporting the fins; have an operculum (a cover over the gill), 
which helps them to breathe without having to swim; and are able to see in color, unlike most other fish. 

One of the best-known innovations of this group is the ability to produce endochondral or "replacement" 
bone, by replacing cartilage from within, with bone. This is in addition to the production of perichondral or 
"spongy bone." The effect is to create a relatively lightweight, flexible, "spongy" bone interior, surrounded 
by an outline of dense bone. This is still much heavier and less flexible than cartilage. 

The ocean sunfish is the most massive bony fish in the world, up to 3.33 m (11 ft) in length and weighing 
up to 2,300 kg (5,070 lb). Other very large bony fish include the Atlantic blue marlin, the black marlin, some 
sturgeon species, the giant grouper and the goliath grouper. In contrast, the dwarf pygmy goby measures 
only 1.5 cm (0.6 in). 



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Figure 8: An ocean sunfish, the most massive bony fish in the world, up to 11 ft in length and 5,070 lb in 
weight! 

(Source: http://commons.wikimedia.Org/lmage:Mola_mola_ocean_sunfish_Monterey_Bay_Aquarium_2.jpg, 
Photographer: Fred Hsu, License: GNU-FDL) 

Why Fish are Important 

Now that you have some understanding of the general features offish, you might come up with some ways 
in how fish are important. Of course, what comes to mind right away is their use for food. In fact, people 
from around the world either fish them from the wild or farm them in much the same way as cattle or chickens 
(aquaculture). Fish are also exploited for recreation, through angling and fishkeeping, and are commonly 
exhibited in public aquaria. 



303 




Figure 9: Workers harvest catfish from the Delta Pride Catfish farms in Mississippi. 

(Source: http://commons.wikimedia.Org/lmage:Delta_Pride_Catfish_farm_harvest.jpg, Photographer: Ken 
Hammond, License: PD - US Federal Government) 

Fish also have an important role in many cultures and art through the ages, ranging as widely as deities 
and religious symbols to subjects of books and popular movies. For example, such deities that take the form 
of a fish are Ikee-Roa of the Polynesians, Dagon of various ancient Semitic peoples, and Matsya of the 
Dravidas of India. Fish have been used figuratively in many different ways, for example the ichthys used by 
early Christians to identify themselves and the fish as a symbol of fertility among Bengalis. 



304 



Figure 10: Fish play an important role in many cultures, including art, through the ages. Here is a still life 
offish, eels, and fishing nets, by Johannes Fabritius. 

(Source: http://commons.wikimedia.org/lmage:Johannes_Fabritius_-_Still_life_of_fish,_eels,_and_fish- 
ing_nets.jpg) 

In literature, legends of half-human, half-fish mermaids are featured in stories of Hans Christian Anderson 
and fish feature prominently in The Old Man and the Sea. Fish and other fanciful fish also play a major role 
in such movies as Splash, Jaws, Shark Tale, and Finding Nemo. 

Lesson Summary 

The general traits offish help adapt them for living in an aquatic environment, mostly for swimming, and 
also for extracting oxygen. 

Fish are typically ectothermic, although some show warm-blooded adaptations. 

Jawless fish, the Agnatha, also have some other common features. 

Fish with jaws comprise both the cartilaginous fish and the bony fish. 

The cartilaginous fishes, or Chondrichthyes, include the sharks, rays, skates and chimaera. 

The bony fish, or Osteichthyes, is the largest group of vertebrates in existence today and have certain 
traits in common. 

Fish are important economically, recreationally and culturally. 

Review Questions 

1 . What are the general traits of fish? (Beginning) 

1. What are some exceptions to the general traits offish? (Intermediate) 

1 . Mudskippers are an example of a fish species that must absorb oxygen across the skin, instead of via 
gills, since they spend much of their time out of water. Describe an environmental situation in which air 
breathing would be of great use to a fish species. (Challenging) 

1 . What are the characteristics of jawless fish? (Beginning) 

1 . What is one structure only found in cartilaginous fishes and what is its function? (Intermediate) 



305 



1 . What are some reasons why it would be an advantage for fish to be endothermic? (Challenging) 
1 . List some ways that fish are important. (Beginning) 

Further Reading I Supplemental Links 

Unabridged Dictionary, Second Edition. Random House, New York, 1998. 

http://kids.nationalgeographic.com/Animals 

http://www.fws.gov/educators/students.html 

http://www.igfa.org/kidshome.asp 

http://www.pbs.org/emptyoceans/educators/activities/fish-youre-eating.html 

http://en.wikipedia.org 

Vocabulary 

agnatha A superclass of jawless fish, belonging to the phylum Chordata, subphylum 

Vertebrata. 

aquaculture The raising of aquatic plants and animals, especially seaweed, shellfish 

and other fish, in environments either natural or with controlled freshwater 
or marine conditions. 

barbels A thin structure on the external part of the head, such as the jaw, mouth or 

nostrils, of certain fishes. 

chondrichthyes The group of cartilaginous fishes, containing sharks, rays, skates and chi- 

maeras. 

ectothermic cold-blooded 

osteichthyes Contains all the bony fish, divided into the ray-finned and lobe finned fish. 

ovoviviparous reproduction The eggs develop inside the mother's body after internal fertilization, and 

depend on the yolk for most of the nutrition; the young are born alive. 

placoid Platelike, as in the scales of sharks. 

Review Answers 

1 . Fish have scales, two sets of paired fins and several unpaired fins. They are typically ectothermic, have 
a streamlined body, extract oxygen from the water using gills, and lay eggs that are fertilized internally 
or externally. 

1. Tuna, swordfish and some species of sharks show some warm-blooded adaptations, and are able to 
raise their body temperature significantly above that of the ambient water around them. Some species 
offish, like eels and rays, have a slower, but more maneuverable swimming style. Moray eels have a 
naked skin surface. 

1 . This would be of use to fish that inhabit shallow, seasonally variable waters, where the oxygen concen- 
tration in the water may decline at certain times of year. At such times, fishes dependent solely on the 
oxygen in the water, such as perch and cichlids, will quickly suffocate, but air-breathing fish can survive 
for much longer, in some cases in water that is little more than wet mud. 

1 . In addition to having no jaws, Agnatha have no paired fins, have a notochord in both larvae and adults, 
and have seven or more paired gill pouches. They have a light sensitive pineal eye, no identifiable 
stomach, and have external fertilization. They are ectothermic, with a cartilaginous skeleton, and a heart 
with two chambers. 



306 



1. The Leydig's Organ is one of the structures to produce red blood cells, since cartilaginous fish do not 
have bone marrow. 

1. Endothermy, though metabolically costly, is thought to provide advantages such as increased contractile 
force of muscles, higher rates of central nervous system processing, and higher rates of digestion. 

1. They are used for food; for recreation, as in angling and fishkeeping; and have played a role in many 
cultures, as deities in religions and as religious symbols, among others. 

Points to Consider 

• Juvenile bichirs, a type offish, have external gills, a very primitive feature that they hold in common with 
larval amphibians. Think about how the external gills could be a transition between internal gills and 
lungs? 

• Lungfish and bichirs have paired lungs similar to those of tetrapods and must rise to the water's surface 
to gulp fresh air through the mouth and pass spent air out though the gills. Discuss how lungfish could 
be similar to and different from tetrapods in the way they breathe? 

• The structure, the pineal body, located in the brain, performs many different functions including detecting 
light, maintaining circadian rhythms and controlling color changes. What structures could perform similar 
functions in amphibians, as a result of living on land? 



Amphibians 

Lesson Objectives 

• Describe amphibian traits. 

• List the features of salamanders. 

• Compare and contrast frogs and toads with other amphibians. 

• Describe the roles of amphibians. 

Check Your Understanding 

1 . What are some adaptations that amphibians, like fish, have for living in the water? 

2. What are the characteristics that amphibians share with all other vertebrates? 

Introduction 

What group of animals begins its life in the water, but then spends most of its life on land? You were right, 
if you guessed amphibians. Amphibians are a group of vertebrates that has adaptations for both aquatic 
and terrestrial lifestyles. Evolutionary, their ancestors made the transition from the sea to land. They comprise 
approximately 6,000 species of various body types, physiology, and habitats, ranging from tropical to sub- 
arctic regions. 

Characteristics of Amphibian 

Amphibians are ectothermic vertebrates, belonging to the class Amphibia and consist of three orders: 
Urodela, containing the salamanders and newts; Anura, consisting of frogs and toads; and Apoda, containing 
the caecilians. The larvae are typically aquatic and breathe using gills. The adults are typically semiterrestrial 
and breathe both through moist skin and by lungs. 

For the purposes of reproduction, most amphibians are bound to fresh water. Although there are no true 
seawater amphibians, a few tolerate brackish (slightly salty) water. Some species do not need any water 



307 



whatsoever, and several species have also adapted to arid and semi-arid environments, but most still need 
water to lay their eggs. 

In general, the life cycle of amphibians begins with a shell-less egg stage, usually laid the previous winter 
in a pond. A larval stage follows in which the organism is legless, fully aquatic and breathes with exterior 
gills. After hatching, the larvae start to transform gradually (metamorphosis) into the adult's appearance, 
including loss of gills, growth of four legs, and the ability to live in a terrestrial environment. 

Adaptations for living in a terrestrial environment include replacement of gills with another respiratory organ, 
such as lungs; a development of glandular (containing cells, a group of cells, or an organ producing a secre- 
tion) skin to avoid dehydration, and the development of eyelids and adaptation to vision outside the water. 
An eardrum also develops that separates the external ear from the middle ear and, in frogs and toads, the 
tail disappears. 

Salamanders 

This is a group of approximately 500 species of amphibians, typically characterized by slender bodies, short 
legs, and long tails, and most closely related to the caecilians, little known legless amphibians. Having moist 
skin, salamanders rely on habitats in or near water or under some protection on moist ground, often in a 
swamp. Some species are aquatic throughout life, some are aquatic intermittently and some are entirely 
terrestrial as adults. 




Figure 1 : The marbled salamander, Ambystoma opacum, shows the typical salamander body plan: slender 
body, short legs, long tail and moist skin. 

(Source: http://commons.wikimedia.Org/lmage:A_opacum_USGS.jpg, License: Public Domain) 



308 




Figure 2: A species of African caecilian, Boulengerula taitanus, a legless amphibian, most closely related 
to the salamanders. 

(Source: http://commons.wikimedia.0rg/lmage:Boulengerula_taitanus_2.jpg, License: GNU-FDL) 

Respiration varies among the different species of salamanders; in those that retain lungs, respiration occurs 
through the gills as water passes over the gill slits. Some terrestrial species have lungs that are used in 
respiration in a similar way as in mammals. Other terrestrial salamanders lack both lungs and gills and ex- 
change gases through their skin. This is known as valarian respiration, in which the capillary beds are 
spread throughout the epidermis. 

Hunting prey is another unique aspect of salamanders. Muscles surrounding the hyoid bone contract to 
create pressure and "shoot" the hyoid bone out of the mouth along with the tongue. The tip of the tongue 
has mucus which creates a sticky end to which the prey is attached and captured. Muscles in the pelvic region 
are then used to bring the tongue and hyoid back to their original positions. Another trait, unique among 
vertebrates, is the ability to regenerate lost limbs, as well as other body parts, in a process known as ecdysis. 

Salamanders are found in most moist or arid habitats in the northern hemisphere. They are generally small, 
but some can reach 30 cm (a foot) or more, as in the mudpuppy of North America. In Japan and China, the 
giant salamander reaches 1 .8 m (6 ft) and weighs up to 30 kg (66 lb). 




309 



Figure 3: The Pacific giant salamander can reach up to 6 ft in length and 66 lb in weight. 

(Source: http://commons.wikimedia.Org/lmage:Pacific_Giant_Salamander.jpg, Photographer: Mat Honan, 
License: CCA 2.0) 

The order Urodela, containing the salamanders and newts, is divided into three suborders. These consist 
of the giant salamanders (including the hellbender and Asiatic salamanders), advanced salamanders (in- 
cluding lungless salamanders, mudpuppies, and newts), and sirens. 

Frogs and Toads 

Frogs and toads are amphibians in the order Anura. A distinction is often made between frogs and toads 
on the basis of their appearance, caused by the convergent adaptation among so-called toads to dry en- 
vironments (leathery skin for better water retention and brown coloration for camouflage), but this distinction 
has no taxonomic basis. One family, Bufonidae, is exclusively given the common name "toad," but many 
species from other families are also called "toads." 




Figure 4: A species of toad, showing typical characteristics of leathery and warty skin, and brown coloration. 

(Source: http://commons.wikimedia.Org/lmage:Toad15.jpg, License: Public Domain) 

Frogs are distributed from the tropics to subarctic regions, but most species are found in tropical rainforests. 
Consisting of more than 5,000 species (about 88% of amphibian species are frogs), they are among the 
most diverse groups of vertebrates. Frogs range in size from 10 mm (less than Y 2 in) in species in Brazil 
and Cuba to the 300 mm (1 ft) goliath frog of Cameroon. 

Adult frogs are characterized by long hind legs, a short body, webbed digits, protruding eyes and no tail. 
They also have a three-chambered heart, which they share with all tetrapods except birds and mammals. 
Most frogs have a semi-aquatic lifestyle, but move easily on land by jumping or climbing. They typically lay 
their eggs in puddles, ponds or lakes, and their larvae, or tadpoles, have gills and develop in water. 

The reliance of frogs on an aquatic environment for the egg and tadpole stages gives rise to a variety of 
mating behaviors that include the calls used by the males of most species to attract females to the bodies 
of water chosen for breeding. Frogs are most noticeable by these calls, which can occur during the day or 
night. 

Frogs are usually well suited to jumping with long hind legs and elongated ankle bones. They have a short 
vertebral column, with no more than ten free vertebrae, followed by a fused tailbone. Skin hangs loosely on 
the body because of the lack of loose connective tissue(tissue that surrounds, supports, or connects organs, 
other tissues, etc.). Skin texture varies, either smooth, warty or folded. 

Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from 
translucent to opaque. Frogs have a tympanum, involved in hearing, on each side of the head, and in some 
species, is covered by skin. 



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Adult frogs are carnivorous and eat mostly arthropods, annelids and gastropods. Adults have a ridge of very 
small cone teeth, called maxillary teeth, around the upper edge of the jaw and they have what are called 
vomerine teeth on the roof of the mouth. Since they don't have teeth on their lower jaw, frogs usually swallow 
their food whole, and use the teeth they do have to hold the prey in place. Toads do not have any teeth, 
and so they must swallow their prey whole. 

Roles of Amphibians 

Frogs are raised commercially as a food source (frog legs are a delicacy in China, France, the Philippines, 
northern Greece and the American south, especially Louisiana). They are used in cloning research and 
other branches of embryology, because they lack egg shells, and therefore facilitate observations of early 
development. The African clawed frog or platanna (Xenopus laevis) is used as a model organism (a species 
that is extensively studied to understand certain biological phenomena) in developmental biology, because 
it is easy to raise in captivity and has a large and easily manipulated embryo. Many Xenopus genes have 
been identified, isolated, and cloned as a result. 

Many environmental scientists believe that amphibians, including frogs, are excellent biological indicators 
of broader ecosystem health because of their intermediate position in food webs, their permeable skins, 
and typically biphasic life (aquatic larvae and terrestrial adults). 

Amphibians also figure prominently in folklore, fairy tales and popular culture. Numerous legends have de- 
veloped over the centuries around the salamander (its name originates from the Persian, for "fire" and "within) 
, many related to fire. This connection likely originates from the tendency of many salamanders to dwell inside 
rotting logs. When placed into the fire, salamanders would escape from the logs, lending to the belief that 
the salamander was created from flames. 

Associations of the salamander with fire appear in the Talmud (a collection of Jewish law and tradition) and 
the Hadith (a traditional account of things said or done by Muhammad or his companions), as well as in the 
writings of Conrad Lycosthenes ( a sixteenth century humanist and encyclopedist), Benvenuto Cellini (a 
sixteenth century Italian goldsmith, painter, sculptor, musician, and soldier), science fiction authors Ray 
Bradbury and David Weber, Paracelsus (a fifteenth century alchemist, physician, and astrologer) and 
Leonardo da Vinci. 

In other representations in popular culture, salamanders are known as minor snake demons according to 
some folklore; they, and frogs, appear as some characters in video games; salamanders appear in anime 
series, and they were even the focus of a dance craze (the Salamander Homp) in the early 1980's. Frogs 
tend to be portrayed as benign, ugly, and clumsy, but with hidden talents. Examples include Michigan J. 
Frog, The Frog Prince, and Kermit the Frog. 

The Moche people of ancient Peru worshipped animals and often depicted frogs and toads in their art. The 
toad also appears as symbol and in story in Vietnamese culture. 

Lesson Summary 

• Amphibians have adaptations for both aquatic, including gills, and terrestrial, including lungs and moist 
skin, lifestyles. 

• Most amphibians are bound to water for reproduction. 

• Development includes a shell-less egg, larval stage and adult. 

• Salamanders have some unique features, including the use of the hyoid bone in hunting prey, and the 
process of ecdysis. 

• Adult frogs and toads have features for living in the water (such as webbed digits) and for living on the 
land (such as long hind legs for jumping). 

• Frogs are well known for their mating calls, which are used to attract females to aquatic breeding grounds. 



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• Amphibians play a role economically as a food source; are used in various types of biological research, 
can serve as indicators of ecosystem health, and figure prominently in folklore and popular culture. 

Review Questions 

1 . Describe the general traits of amphibians. (Beginning) 

1 . Describe the life cycle of amphibians. (Beginning) 

1 . What are some adaptations of amphibians for living in a terrestrial environment? (Intermediate) 

1. A frog's skin must remain moist at all times in order for oxygen to pass through the skin and into the blood. 
Why does this fact make frogs susceptible to many toxins in the environment? (Challenging) 

1. The permeability of a frog's skin can result in water loss. What adaptations would benefit a frog by 
counteracting this water loss? (Challenging) 

1 . Name how one feature of frog development lends itself to research applications. (Beginning) 

1. Amphibians have a number of adaptations which make it easy for them to avoid predation. Describe 
some of these. (Challenging) 

Further Reading I Supplemental Links 

Unabridged Dictionary, Second Edition. Random House, New York, 1998. 

http://en.wikipedia.org/wiki 

http://kids.nationalgeographic.com/Animals 

http://amphibiaweb.org 

http://helpafrog.org 

http://www.amphibianark.org/yearofthefrog.htm 

http://www.epa.gov/gmpo/education/photo/amphibians.html 

Vocabulary 

convergent adaptation The appearance of similar traits in groups of animals that are evolutionary unre- 
lated to each other. 

ecdysis The ability to regenerate lost limbs, as well as other body parts. 

hyoid bone A U-shaped bone at the root of the tongue; in salamanders it is used to help catch 

prey. 

tympanum Equivalent to the middle ear; used in hearing. 

valarian respiration Respiration in which the capillary beds are spread throughout the epidermis, so 

that gases can be exchanged through the skin. 

Review Answers 

1. Amphibians are ectothermic; the larvae are typically aquatic with gills and the adults are typically 
semiterrestrial and breathe using moist skin and lungs. Most species lay their eggs in water. 

1. A shell-less egg stage; followed by the larval stage, with no legs, fully aquatic, and exterior gills, metamor- 
phosis into the adult, including loss of gills, growth of four legs, and the ability to live in a terrestrial envi- 
ronment. 



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1 . Loss of gills and breathing via lungs, moist skin, or other means; growth of four legs; glandular skin, to 
avoid dehydration; development of eyelids and adaptation of vision to living on land; adaptations for 
jumping in frogs and toads. 

2. Some of these toxins can similarly dissolve in the layer of water and be passed into the bloodstream. 

1. A leathery skin, as in toads, helps prevent water loss. Behaviors can also help. For example, nocturnal 
activity limits exposure to higher temperatures. Resting in water-conserving positions, as toes and fingers 
tucked under the body and chin, respectively, with no gap between the body and substrate, also helps. 
Frogs can also rest in large groups, touching the skin of neighboring frogs, thus reducing the amount of 
skin exposed to the air or dry surface. 

1 . Lack of egg shells makes frogs ideal for research in embryology, because the stages of early development 
can easily be observed. 

1 . Locomotion, such as jumping, climbing, swimming, even gliding; camouflage, including color change and 
features such as warts and skin folds on ground-dwelling species and smooth skin in arboreal species; 
nocturnal behavior; frogs and toads with toxins that make them unpalatable; synchronous reproduction, 
thus overwhelming the actions of predators; egg and larval adaptations, such as presence of toxins, and 
guarding of eggs and parental care of larvae. 

Points to Consider 

• Future studies of molecular genetics should soon provide further insights to the evolutionary relationships 
among frog families. These studies will also clarify relationships among families belonging to the rest of 
vertebrates as well. 

• Toxins obtained from some frog species may have potential as therapeutic drugs. The alkaloid epibatidine, 
a painkiller 200 times more potent than morphine, is found in some species of poison dart frogs. Other 
chemicals isolated from frog skin may offer resistance to HIV infection. As we will see in the next lesson, 
reptiles also possess chemicals and unique genes that are useful for medical purposes. 

• Although care of offspring is poorly understood in frogs, it is estimated that up to 20% of amphibian 
species care for their young, and that there is a great diversity of parental behaviors. As you begin to 
examine the reproductive system of reptiles in the next lesson, think about what kinds of parental behaviors 
reptiles might have and how they compare to that of amphibians. 



Reptiles 

Lesson Objectives 

List reptile traits. 

Describe the general features of lizards and snakes. 

List the characteristics of alligators and crocodiles. 

Describe the traits of turtles. 

Explain the importance of reptiles. 

Check Your Understanding 

1 . What are some adaptations for living on land that are present in the amphibians? 



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2. What features present in amphibians are also useful to reptiles for an aquatic lifestyle? 

Introduction 

While some types of reptiles, like snakes, alligators, and crocodiles, often have a bad reputation due to their 
venom, as in snakes, or their aggressive behavior, as in all three groups, reptiles are important both ecolog- 
ically and economically, as we will see later in this lesson. They also possess some unique traits and inter- 
esting behaviors, which we will also explore in greater detail. 

Reptiles are tetrapods and amniotes, whose embryos are surrounded by an amniotic membrane. Modern 
reptiles inhabit every continent with the exception of Antarctica, and are represented by four living orders: 
Squamata (lizards, snakes and amphisbaenids or "worm-lizards"), Crocodilia (crocodiles, gharials, caimans, 
and alligators) Testudines (turtles and tortoises) and Sphenodontia (tuatara). 




Figure 1: An Indian gharial crocodile. 

(Source: http://commons.wikimedia.Org/lmage:lndian_Gharial_Crocodile_Digon3.jpg, Photographer: Jonathan 
Zander, License: CCA-BY-SA 2.5, GNU-FDL) 




Figure 2: A tuatara. 



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(Source: http://commons.wikimedia.Org/lmage:Tuatara_southland_museum_invercargill_new_zealand.jpg, 
License: Public Domain) 

Traits of Reptiles 

Reptiles are air-breathing, cold-blooded vertebrates that have skin covered in scales. The majority of species 
are oviparous (egg-laying) although certain species of squamates are capable of giving birth to live young. 
This is achieved, either by ovoviviparity (egg retention within the female until birth), or viviparity (offspring 
born without use of calcified eggs). 

Many of the viviparous species feed their fetuses through various forms of placenta, similar to those of 
mammals, with some providing initial care for their hatchlings. The degree of viviparity varies: some species 
simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the 
yolk, while still others lack any yolk and provide all nutrients via a placenta. 

All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail, where waste 
is also eliminated. Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion 
(the innermost of the embryonic membranes), chorion (the outermost of the membranes surrounding the 
embryo) and allantois (a vascular embryonic membrane) are present during embryonic life. There are no 
larval stages of development. 

Most reptiles reproduce sexually, although six families of lizards and one snake are capable of asexual re- 
production. In some species of squamates, a population of females is able to produce a unisexual diploid 
clone of the mother. This asexual reproduction called parthenogenesis occurs in several species of gecko, 
and is particularly widespread in the teiids and lacertids. 

Extant reptiles range in size from the newly-discovered Jaragua Sphaero, at 1 .6 cm (0.6 in), to the saltwater 
crocodile, at up to 7 m (23 ft). 

Most reptiles have a closed circulatory system with a three-chambered heart consisting of two atria and one 
ventricle. All reptiles breathe using lungs, although aquatic turtles have developed more permeable skin, 
and some species have modified their cloacas to increase the area for gas exchange. Excretion is performed 
mainly by two small kidneys. 

The reptilian brain is similar to that of amphibians, except the cerebrum and cerebellum are slightly larger. 
Most typical sense organs are well developed with certain exceptions most notably the snakes lack of external 
ears (middle and inner ears are present). All reptilians have advanced visual depth perception compared 
to other animals. 

Lizards and Snakes 

Lizards and snakes belong to the largest recent order of reptiles (Squamata). Members of the order are 
distinguished by their skin, which bears horny scales or shields. They also possess movable quadrate bones, 
making it possible to move the upper jaw relative to the braincase. This is particularly visible in snakes, 
which are able to open their mouths very widely to accommodate comparatively large prey. 



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Figure 3: A corn snake swallowing a mouse. 

(Source: http://commons.wikimedia.0rg/lmage:Corn_snake_swallowing_cropped.jpg, Photographer: 
Bachrach44, License: Public Domain) 

Lizards are a large and widespread group of reptiles, with nearly 5,000 species, ranging across all continents 
except Antarctica. Most lizards have four limbs, external ears, movable eyelids, a short neck, a long tail, 
and are insectivores. Many can shed their tails in order to escape from predators. 

Vision, including color vision, is particularly well developed in lizards, and most communicate with body 
language, bright colors, or pheromones. Adults range from a few cm ( 1 in) in length (some Caribbean 
geckos) to nearly 3 m (10 ft), although most species are less than 220 g (0.5 lb). 




Figure 4: A Komodo dragon, the largest of the lizards, attaining a length of 1 ft. 

(Source: http://commons.wikipedia.0rg/lmage:Varanus_komodoensisl.jpg, License: GNU-FDL) 

Encompassing 40 families, there is tremendous variety in color, appearance and size of lizards. Most lizards 
are oviparous, although a few species are viviparous. Many are also capable of regeneration of lost limbs 
or tails. Almost all lizards are carnivorous, although most are so small that insects are their primary prey. A 
few species are omnivorous or herbivorous, and others have reached sizes where they can prey on other 
vertebrates, such as birds and mammals. 

Many lizards are good climbers or fast sprinters. Some can run bipedally, such as the collared lizard, and 
some, like the basilisk, can even run across the surface of water to escape. Many lizards can change color 
in response to their environments or in times of stress The most familiar example is the chameleon, but 
more subtle color changes occur in other lizard species, such as the anole, as well. 



316 




Figure 5: A species of lizard, showing general body form and camouflage against background. 

(Source: http://commons.wikimedia.Org/lmage:Lizard_on_a_rock_on_Park_Avenue_in_Arches_NP2.jpg) 

Some lizard species, including the glass lizard and flap-footed lizards, have lost their legs or reduced them 
to the point they are non-functional. However, some vestigial structures remain. While some legless lizards, 
like flap-footed lizards, are similar in appearance to snakes, they can be distinguished by such features as 
their external ears. 

All snakes are carnivorous and can be distinguished from legless lizards by lack of eyelids, limbs, external 
ears, and vestiges of forelimbs. The 2,700+ species of snakes occur in every continent except Antarctica 
and range in size from the tiny, 10 cm (4 in) long thread snake to pythons and anacondas over 5 m (17 ft) 
long. In order to accommodate snakes' narrow bodies, paired organs (such as kidneys) appear one in front 
of the other instead of side by side. 




Figure 6: A species of anaconda, one of the largest snakes, getting as long as 1 7 ft. 

(Source: http://commons.wikimedia.Org/lmage:AnacondaJaune_34.jpg) 

While venomous snakes comprise a minority of the species, some possess potent venom capable of causing 
painful injury or death to humans. However, snake venom is primarily for killing and subduing prey rather 
than for self-defense. All snakes are strictly carnivorous, eating small animals including lizards, other snakes, 
small mammals, birds, eggs, fish, snails or insects. 



317 



Because snakes cannot bite or tear their food to pieces, prey must be swallowed whole. The body size of 
a snake has a major influence on its eating habits. The snake's jaw is one of the most unique jaws in the 
animal kingdom. Snakes have a very flexible lower jaw, the two halves of which are not rigidly attached, 
and numerous other joints in their skull, allowing them to open their mouths wide enough to swallow their 
prey whole. 

Some snakes have a venomous bite, which they use to kill their prey before eating it; others kill their prey 
by constriction, and still others swallow their prey whole and alive. After eating, snakes become dormant 
while the process of digestion takes place. The process is highly efficient, with the snake's digestive enzymes 
dissolving and absorbing everything but the prey's hair and claws. 

Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, 
keeled or granular . Snakes' eyelids are transparent "spectacle" scales which remain permanently closed. 
In the shedding of scales, or molting, the complete outer layer of skin is shed in one layer. Molting replaces 
old and worn skin, allows the snake to grow and helps it get rid of parasites such as mites and ticks. 




Figure 7: A close up of snake scales of a banded krait, Bungarus fasciatus, showing Black and yellow alter- 
nating bands and spaces between scales. 

(Source: http://commons.wikimedia.Org/lmage:AB_053_Banded_Krait.jpg) 




Figure 8: A northern water snake shedding its skin. 

(Source: http://commons.wikimedia.0rg/lmage:Neradia_sipedon_shedding.jpg, License: CC-BY-SA 2.5) 

Although a wide range of reproductive modes are used by snakes, all snakes employ internal fertilization, 
accomplished by means of paired, forked hemipenes, which are stored inverted in the male's tail. Most 



318 



species of snakes lay eggs and most species abandon them shortly after laying. 

Alligators and Crocodiles 

Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles belonging to 
Crocodilia are the closest living relatives of birds, as the two groups are the only known living descendants 
of the Archosauria, a subclass of reptiles, including the dinosaurs. The basic crocodilian body plan is a very 
successful one that has changed little over time; modern species closely resemble their Cretaceous ancestors 
of 84 million years ago. Crocodilians have a flexible semi-erect (semi-sprawled) posture. They can walk in 
low, sprawled "belly walk," or hold their legs more directly underneath them to perform the "high walk." Most 
other reptiles can only walk in a sprawled position. 




Figure 9: Two Nile crocodiles, showing the basic crocodilian body plan. 

(Source: http://commons.wikimedia.0rg/lmage:NileCrocodile.jpg., License: CC-BY-SA 2.0) 

All crocodilians have, like humans, thecodont dentition, (teeth set in bony sockets), but unlike mammals, 
they replace their teeth throughout life. Crocodilians also have a secondary bony palate that enables them 
to breathe when partially submerged, even if the mouth is full of water. Their internal nostrils open in the 
back of their throat, where a special part of the tongue called the "palatal valve" closes off their respiratory 
system when they are underwater, allowing them to breathe when submerged. 

Crocodiles and gharials (large crocodilians having elongated jaws) have modified salivary glands on their 
tongue (salt glands), which are used for excreting excess salt ions from their bodies. Crocodilians are often 
seen lying with their mouths open, a behavior called gaping. One of its functions is probably to cool them 
down, but it may also have a social function. 

Like mammals and birds and unlike other reptiles, crocodiles have a four-chambered heart; however, unlike 
mammals, oxygenated and deoxygenated blood can be mixed. Crocodilians are known to swallow stones, 
known as gastroliths, which act as a ballast in addition to aiding post-digestion processing of their prey. The 
crocodilian stomach is divided into two chambers, the first is powerful and muscular, like a bird gizzard, 
where the gastroliths are found. The other stomach has the most acidic digestive system of any animal and 
can digest mostly everything from their prey: bones, feathers and horns. 

The sex of developing crocodilians is determined by the incubation temperature of the eggs. This means 
crocodilians do not have genetic sex determination, but instead have a form of environmental sex determi- 
nation, which is based on the temperature that embryos are subjected to early in their development. 

Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more advanced than 
those of other reptiles. As in many other aquatic or amphibian tetrapods, the eyes, ears, and nostrils are all 
located on the same plane. They see well during the day and may even have color vision, plus the eyes 



319 



have a vertical, cat-like pupil, which gives them excellent night vision. A third transparent eyelid, the nicti- 
tating membrane, protects their eyes underwater. 

While birds and most reptiles have a ring of bones around each eye which supports the eyeball (the sclerotic 
ring), the crocodiles lack these bones, just like mammals and snakes. The eardrums are located behind the 
eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when 
they dive, preventing water from entering their external head openings. The middle ear cavity has a complex 
of bony air-filled passages and a branching Eustachian tube. Eustachian tubes will be discussed in the 
chapter titled Controlling the Body. 

The upper and lower jaws are covered with sensory pits, which encase bundles of nerve fibers that respond 
to the slightest disturbance in surface water. Thus they can detect vibrations and small pressure changes 
in water, making it possible for them to detect prey, danger and intruders even in total darkness. 

Turtles 

Turtles are reptiles of the order Testudines, most of whose body is shielded by a special bony or cartilaginous 
shell developed from their ribs. About 300 species are alive today and some are highly endangered. Turtles 
cannot breathe in water, but can hold their breath for various periods of time. Like other reptiles, turtles are 
poikilothermic (or "of varying temperature"). Like other amniotes, they breathe air and don't lay eggs un- 
derwater, although many species live in or around water. 

The largest chelonian (all living species) is the great leatherback sea turtle, which reaches a shell length of 
200 cm (7 ft) and can reach a weight of over 900 kg (2,000 lb). Freshwater turtles are generally smaller, but 
the largest species, the Asian softshell turtle, has been reported up to 200 cm (7 ft). The only surviving giant 
tortoises are on the Seychelles and Galapagos Islands and can grow to over 130 cm (4 ft) in length and 
weigh about 300 kg (670 lb). 




Figure 10: The largest living chelonian, the leatherback turtle, which can reach up to 7 ft in length and over 
2,000 lb. 

(Source: http://commons.wikimedia.Org/lmage:LeatherbackTurtle.jpg, License: Public Domain) 



320 




Figure 11: A Galapagos giant tortoise, pictured here, can grow to over 4 ft in length and weigh about 670 
lb. 

(Source: http://commons.wikimedia.Org/lmage:67705078_e9550fab12_o.jpg) 

The smallest turtle is the speckled padloper tortoise of South Africa, measuring no more than 8 cm (3 in) in 
length, and weighing about 140 g (5 oz). Turtles are broken down into two groups, according to how they 
evolved a solution to the problem of withdrawing their neck into the shell: the Cryptodira, which can draw 
their neck in while contracting it under their spine, and the Pleurodira, which contract their neck to the side. 

Most turtles that spend most of their life on land have their eyes looking down at objects in front of them. 
Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the 
head. These species of turtles can hide from predators in shallow water where they lie entirely submerged 
except for their eyes and nostrils. Sea turtles possess glands near their eyes that produce salty tears that 
rid their body of excess salt taken in from the water they drink. 




Figure 12: A species of sea turtle, showing placement of eyes, shell shape, and flippers. 

(Source: http://commons.wikimedia.Org/lmage:Sea_Turtle.jpg, License: Public Domain) 

Turtles are thought to have exceptional night vision due to the unusually large number of rod cells in their 
retinas. Turtles have color vision with a wealth of cone subtypes with sensitivities ranging from the near ul- 
traviolet to red. (For a description of rods and cones, see chapter titled Controlling the Body). Turtles have 
a rigid beak and use their jaws to cut and chew food. Instead of teeth, the upper and lower jaws of the turtle 
are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their 
prey. Herbivorous turtles have serrated-edged ridges that help them cut through tough plants. 

Although many turtles spend large amounts of their lives underwater, all turtles and tortoises breathe air, 
and must surface at regular intervals to refill their lungs. They can also spend much of their lives on dry land. 



321 



Turtles lay eggs, like other reptiles, and which are slightly soft and leathery. The eggs of the largest species 
are spherical, while the eggs of the rest are elongated. In some species, temperature determines whether 
an egg develops into a male or female. Large numbers of eggs are deposited in holes dug into mud or sand. 
They are then covered and left to incubate by themselves. When the turtles hatch, they squirm their way to 
the surface and head toward the water. 

Importance of Reptiles 

The chief impact of reptiles, such as lizards, on humans is positive as they are significant predators of pest 
species. Snakes are also very useful rat exterminators, for example, in the Irula villages of India. 

Reptiles can be important as food sources: green iguanas are eaten in Central America, the tribals of "Irulas" 
from Andhra Pradesh and Tamil Nadu in India are known to eat some of the snakes they catch, Cantonese 
snake soup is consumed by local people in the fall to prevent colds, cooked rattlesnake meat is commonly 
consumed in parts of the Midwestern United States, and turtle soup is widely consumed. 

Reptiles also make good pets. Numerous lizard species are prominent in the pet trade. In the Western world, 
some snakes, especially docile species such as the ball python and corn snake, are kept as pets. Turtles, 
particularly small terrestrial and freshwater turtles, are also commonly kept as pets. Among the most popular 
are the Russian tortoises, Greek spur-thighed tortoises and red-ear sliders (or terrapin). 

For medical and scientific research, snake venom collected by the "Irulas" is used for producing life-saving 
antivenin and for other medicinal products. Observations about turtle longevity (the liver, lungs and kidneys 
of a centenarian turtle are virtually indistinguishable from those of its immature counterpart) have inspired 
genetic researchers to begin examining the turtle genome for longevity genes. 

Finally, reptiles play a significant role in folklore, religion and popular culture. Lizard symbology plays important, 
though rarely predominant roles in some cultures (e.g. Tarrotarro in Australian mythology). The Moche 
people of ancient Peru worshipped animals and often depicted lizards in their art. Crocodilians have starred 
in several science fiction movies such as Lake Placid and DinoCroc. There are also many cultural depictions 
of turtles and tortoises. 

Snakes or serpents (the latter usually referring to a mythic or symbolic snake) are associated with healing 
in the Bible (the account of the brass serpent of Moses) as well as with the devil (the Biblical account of 
Adam and Eve). The periodic renewal, as in the shedding of snake skin, has led to the snake being a symbol 
of healing and medicine, as pictured in the Rod of Asclepius . In Egyptian history, the snake occupies a 
primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It was worshipped as 
one of the gods and was also used for sinister purposes, such as murder of the adversary and ritual suicide 
by the Egyptian queen Cleopatra. Snakes also play a role in Greek mythology, in Indian tradition and religion, 
and in other religions and customs. 




Figure 13: The Rod of Asclepius, where the snake is a symbol of healing and medicine. 



322 



(Source: http://c0mm0ns.wikimedia.0rg/lmage:R0d_0f_asclepius.png, Photographer: Ddcfnc, License: GNU- 
FDL) 

Lesson Summary 

Reptiles are air-breathing, cold-blooded vertebrates characterized by a scaly skin. 

Reptiles have a variety of reproductive systems, with different strategies for providing nutrition to devel- 
oping young. 

Lizards and snakes are distinguished by a unique type of scaly skin and movable quadrate bones. 

There is a tremendous variety in color, appearance and size of lizards, and they have some unique 
adaptations, including regeneration of lost limbs or tails and changing color. 

Snakes are distinguished by lack of eyelids, limbs, external ears and vestiges of forelimbs. 

Snakes have various adaptations for killing and eating their prey. 

Crocodilia have a flexible semi-erect posture, thecodont dentition, replacement of teeth, and a secondary 
bony palate. 

The sex of developing crocodilians is determined by the incubation temperature of the eggs. 

Other crocodilian traits, such as salt glands, nictitating membranes, ear flaps and sensory pits, are 
adaptations for aquatic living. 

Turtles are characterized by a special bony or cartilaginous shell; have specialized adaptations for aquatic 
living, such as eye placement and salt glands, and adaptations for terrestrial living as well (placement 
of eyes and protection of eggs). 

Reptiles play important roles as predators of pest species, food sources, pets, in medical and scientific 
research, and in folklore, religion and popular culture. 

Review Questions 

1 . Describe the general traits of reptiles. (Beginning) 

2. Describe the different types of reproduction in reptiles. (Beginning) 

3. How are snakes distinguished from legless lizards? (Beginning) 

4. Pit vipers, pythons and some boas have infrared-sensitive receptors in deep grooves between the nostril 
and eye. What role might such receptors play? (Challenging) 

5. Name two adaptations of a crocodilian stomach which help it in digestion. (Intermediate) 

6. The shape and structure of a turtle's shell can give its inhabitant advantages for avoiding predators, aid 
in swimming and diving, and for walking on land. Given what you know about a turtle's shell, explain how 
the structure and shape could help the turtle in the above situations. (Challenging) 

Further Reading I Supplemental Links 

Unabridged Dictionary, Second Edition. Random House, New York, 1998. 

http://en.wikipedia.org 

http://kids.nationalgeographic.com/Animals 

http://www.amnh.org/exhibitions/lizards 



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http://teacher.scholastic.com/activities/explorations/lizards/index.htm 

http://www.turtles.org 

http://www.gma.org/turtles 

http://www.kidskonnect.com/content/view/54/27 

http://www.flmnh.ufl.edu/cnhc/cbd.html 

http://home.cfl.rr.com/gatorhole 

Vocabulary 



amniotes 

nictitating mem 
brane 

oviparous 

parthenogene- 
sis 

pheromones 

poikilothermic 
the codont 



Vertebrates whose embryos are surrounded by an amniotic membrane. 
A third transparent eyelid. 

Reproduction involving the laying of eggs. 

A form of asexual reproduction, where the egg develops without fertilization. 

Chemicals released by an animal that influence the behavior or physiology of other indi- 
viduals of the same species. 

Cold-blooded; without the ability to independently warm the blood. 

Where teeth are set in bony sockets. Review Answers 1 . Reptiles are cold-blooded ver- 
tebrates with skin covered in scales. Most reptiles have a closed circulatory system with 
a three-chambered heart. All reptiles breathe using lungs and excretion is performed 
mainly by two small kidneys. Most typical sense organs are well developed. The majority 
of species are oviparous. 2. The majority of species are oviparous although certain species 
of squamates are capable of giving birth to live young. This is achieved either by ovo- 
viviparity or viviparity. Most reptiles reproduce sexually, although six families of lizards 
and one snake are capable of asexual reproduction. 3. Snakes lack eyelids, limbs, external 
ears and vestiges of forelimbs. 4. Infrared sensitivity helps snakes locate nearby prey, 
especially warm-blooded mammals. 5. Crocodilians swallow stones, gastroliths, which 
aid post-digestion processing of the prey. The second chamber of the stomach has a 
very acidic digestive system and can digest mostly everything, including bones, feathers 
and horns. 6. Most tortoises have a large dome-shaped shell that makes it difficult for 
predators to crush the shell between their jaws. One of the few exceptions is the African 
pancake tortoise which has a flat, flexible shell that allows it to hide in rock crevices. Most 
aquatic turtles have flat, streamlined shells which aid in swimming and diving. Tortoises, 
being land-based, have rather heavy shells. In contrast, aquatic and soft-shelled turtles 
have lighter shells that help them avoid sinking in water and swim faster with more agility. 
Points to Consider 

• Some lizards have a dewlap, a brightly colored patch of skin on the throat, which is 
used in displays. What colorful displays do you think are used for courtship in birds 
and mammals? 

• Lizards and snakes use smell to track their prey, using the Jacobson's or vomeronasal 
organ in the mouth, as well as a forked tongue. How do you think this compares to 
the sense of smell in birds and mammals and the structures used for smelling in these 
groups? 

• Like the scales comprising the shell of a turtle, or the cross-section of a tree trunk, 
crocodile osteoderms (small plates of bone under the scales) have annual growth 
rings, and by counting them it is possible to tell their age. Can we determine age in 
the same way in either birds or mammals? 



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14. Birds and Mammals 



Birds 

Lesson Objectives 

• List and describe general traits of birds. 

• Explain how birds are adapted for flight. 

• List different breeding systems in birds and describe nesting, incubation and parental care. 

• Illustrate the diversity of birds with examples of some of the varied groups. 

• Explain how birds are important, both economically and ecologically. 

Check Your Understanding 

1. Birds and reptiles have some traits in common. For example, birds are egg-layers and most reptiles are 
also oviparous. What do the eggs of both groups have in common? 

2. What traits are there in birds as a result of them being warm-blooded? 

Introduction 

We all think we know what a bird is. It seems fairly obvious. But if you were to really stop and think about 
birds, you would be amazed at the diversity of these organisms. From hummingbirds to ostriches, and 
countless varieties in between, birds are amazing creatures. 

It is pretty easy to be aware of birds all around us. From pet birds in our houses to those seen flying and 
perching in the out-of-doors, birds constantly remind us of their diversity in both appearance and habits. 
Birds have special adaptations for flight, including feathers and a lightweight skeleton. They also have a 
wide variety of reproductive strategies among the different types of birds. Let us examine some of their 
principle traits so we can get a better appreciation of what birds can do. 

Characteristics of Birds 

Birds (class Aves) are warm-blooded, vertebrate animals with two legs (bipedal), who lay eggs. They range 
in size from the tiny 2 in (5 cm) Bee Hummingbird to the 9 ft (2.7 m) ostrich (Figure 1). With approximately 
10,000 living species, birds are the most numerous vertebrates with four limbs (tetrapod). They occur in di- 
verse habitats across the globe, ranging from the Arctic to the Antarctic. 



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Figure 1 : The ostrich can reach a height of 9 feet! Pictured here are ostriches with young in Namibia, Africa. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Struthi0_camelus.jpg, Photographer: Susann Eurich, 
License: CCA 2.5) 

Defining characteristics of modern birds include: 

• feathers 

• high metabolism 

• a four-chambered heart 

• a beak with no teeth 

• a lightweight but strong skeleton 

• production of hard-shelled eggs 

The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones 
for grinding food to compensate for the lack of teeth. Birds have forelimbs modified as wings and nearly all 
can fly. Which of the above traits do you think might be of importance to flight? 

Adaptations for Flight 

In comparing birds with other vertebrates, what do you think distinguishes them the most? Of course, in 
most birds flight is the most obvious difference (Figure 2), and birds have adapted their body plan for this 
function. Their skeleton is especially lightweight, with large pneumatic (air-filled) cavities connecting to the 
respiratory system. Cervical, or neck, vertebrae are especially flexible and in birds with flight the sternum 
has a keel, or longitudinal ridge, for the attachment of two large flight muscles: the pectoralis, which encom- 
passes 15% of the bird's total mass, and the supracoracoideus, the primary upstroke muscle for flight. 

What other traits do you think might be important for flight? Of course, feathers are lightweight too and a 
forelimb modified as a wing serves as an aerofoil. This surface is designed to aid in lifting or controlling by 
making use of the air currents through which it moves. A bird's wing shape and size will determine how a 
species flies. For example, many birds have powered, flapping flight at certain times, while at other times 
they soar, using up less energy 



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Figure 2: One bird's flight, as seen in a tern species. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:TernOI.jpg; License: CC-BY-SA Germany 2.0) 

About 60 living bird species are flightless, such as penguins, as were many extinct birds. Flightlessness often 
arises in birds on isolated islands, probably due to limited resources and the absence of land predators. 

Reproduction in Birds 

How do birds reproduce? We are all familiar with the classic chicken egg. So what is involved in the process 
of a bird laying an egg? It all starts with courtship. Courtship involves some type of courtship display, usually 
performed by the male, leading up to the breeding. Most displays involve a type of song and some displays 
are very elaborate and may include dancing, aerial flights, or wing or tail drumming. 

One of the most distinguishing features of bird reproduction is internal fertilization and the laying of eggs. 
The hard-shelled eggs have a fluid-filled amnion, a thin membrane forming a closed sac around the embryo. 
Eggs are usually laid in a nest. How do you think where a bird lays an egg might influence the egg color? 
If an egg is hidden in a hole or burrow, away from predators, then the eggs are most often pale or white. 
Nests in the open have eggs that are camouflaged, thus giving protection against predation (Figure 3). 
However, some species like the ground-nesting nightjars, have pale eggs, but the birds themselves provide 
the camouflage with their feathers. 




Figure 3: Nest and eggs of the common moorhen (Gallinula chloropus), showing camouflaged eggs. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:VVaterhoennest.jpg, Photographer: Ori, License: Public 
Domain) 



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The shape of birds' nests varies quite a lot too. Most create somewhat elaborate nests, consisting of such 
structures as cups, domes, plates, mounds or burrows. The albatross, however, makes a nest that is simply 
a scrape on the ground. Still others, like the common guillemot, do not use nests, instead they lay their eggs 
on bare cliffs. The male emperor penguins are even more elaborate in the care of their eggs: they incubate 
the eggs between their body and feet. 

How else might a bird help protect its young from predators? Most species locate their nests in areas that 
are hidden, in order to avoid predators. Other birds that are large or nest in colonies may build nests in the 
open, since they are more capable of defending their young. 

Young Birds and Parental Care 

Parent birds usually incubate their eggs after the last one has been laid. In the 95% of species which are 
monogamous, (the species pair for the duration of the breeding season or sometimes for a few years or 
until one mate dies) the parents take turns incubating. In polygamous species, where there is more than 
one mate, one parent does all the incubating. 

Brood parasitism, in which an egg-layer leaves her eggs in another individual's nest, is more common among 
birds than any other type of animal. The host bird often accepts and raises the parasite's eggs, at the expense 
of the host's own offspring. 

Some precocial chicks, like those of the Ancient Murrelet (Synthliboramphus antiquus), follow their parents 
out to sea the night after they hatch, in order to avoid land predators. In most species, however, the young 
leave the nest just before, or right after, they can fly, sometimes making it necessary for them to walk until 
they have mastered flying. 

The length and type of parental care varies widely amongst different species of birds. At one extreme in a 
group of birds called the magapodes, parental care ends in hatching. In this case, the newly-hatched chick 
digs itself out of the nest mound without parental help and can take care of itself right away. At the other 
extreme, many seabirds care for their young for extended periods of time, the longest being that of the Great 
Frigatebird, whose chicks take up to six months to fledge (getting parental care until they are ready to fly) 
and then an additional 14 months of being fed by the parents (Figure 4). 




Figure 4: The Great Frigatebird (Fregata minor) adults are known to care for their young for up to 20 months 
after hatching, the longest in a bird species. Here, a young bird is begging for food. 

(Source: http://commons.wikimedia.Org/wiki/File:Great_Frigatebird_chick_begging.JPG; License: Public 
Domain) 

Although male parental care is rare among most groups of animals, in birds it is quite common, more so 
than in any other class of vertebrates. Often, the tasks of defense of territory and nest site, incubation, and 
feeding of chicks are shared between the parents; sometimes one parent undertakes all or most of a partic- 
ular duty. 



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Given all the information so far about birds, what would you say is true about bird diversity? 

Diversity of Birds 

If you guessed that there is a lot of diversity in birds, you guessed correctly. About 10,000 bird species belong 
to 29 different orders, or groups, within the class Aves. They live and breed in most terrestrial habitats and 
on all seven continents. The greatest biodiversity of birds occurs in the tropics. 

There is enormous diversity and a wide range of adaptations of various body parts, such as beaks and feet, 
to the specific habitats of the birds. There is also enormous diversity in the feeding habits of birds. The 
feeding habits of birds is related to the beak shape and size, as well as the foot shape. Birds can be carni- 
vores, insectivores, or generalists, which feed on a variety of foods. Some feed on nectar, such as hum- 
mingbirds. Can you think of some examples of beak shape and size that are adapted to the type of food a 
bird eats? 

Beaks 

For example, parrots and their allies have down-curved, hooked bills, which are well-adapted for cracking 
seeds and nuts, and eating the meat inside (Figure 5). Hummingbirds, on the other hand, have long, thin 
and pointed bills, which are ideal for probing tubular flowers for nectar (Figure 6). Can you also think of 
some different types of bird feet, which might be adapted for different types of habitats? 




Figure 5: The down-curved, hooked bill of a scarlet macaw, a large colorful parrot (Ara macao). 

(Source: http://commons.wikimedia.Org/wiki/lmage:Ara_macao%2C_costa_rica.jpg, License: GNU Free 
Documentation) 




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Figure 6: A long, thin and pointed bill of the Swallow-tailed Hummingbird (Eupetomena macroura). 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Eupetomena_macroural .jpg, License: CC-BY-SA 2.0) 

Feet 

Webbed feet used for swimming or floating, as in waterfowl or gulls and terns, may come to mind (Figure 
7). Other birds, for example, herons, gallinules and rails have four long spreading toes, which are ideal for 
walking delicately in the wetland in which they live (Figure 8). You can now see that you could come up 
with your own ideas for how a particular bird trait is adapted to a specific habitat, food, or other specialized 
requirement. That might even make going out for an outdoor hike more of an adventure! 




Figure 7: The webbed feet of a great black-backed gull (Lams marinus). 

(Source: http://commons.wikimedia.Org/wiki/lmage:Mewa_siodlata_1 .jpg, License: GNU Free Documentation) 




Figure 8: The long spreading toes of an American purple gallinule (Porphyrio martinica). 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:American_Purple_Gallinule_walking.jpg, License: CC- 
BY-SA 2.5) 

Why Birds are Important 

Now that you have some general knowledge about birds, you may want to make a list yourself of how you 
think birds are important. Just think about your daily living and how birds play a role. Do you eat chicken or 
turkey at meals? Do you have pet birds? Do you enjoy going out in your backyard or for a walk and listen 
to the beauty of birdsong or see the iridescent plumage of a bird in the sun? 



330 



What are some other economic uses of birds? One is the harvesting of guano (droppings) for use as fertilizer. 
Another is the use of chickens as an early warning system of diseases, such as West Nile Virus, that affect 
humans. In the latter example, mosquitoes carry the West Nile Virus, bite young chickens and other birds, 
and infect them with the virus. The first human cases of the virus usually follow the first appearance of infected 
birds within three months. Blood samples from young chickens can be tested for the presence of antibodies 
to the virus, and if detected, then this is an early warning that human infection can follow. 

What about how birds can be important ecologically? For example, some nectar-feeding birds are important 
pollinators, and many frugivores, or fruit-eating birds, help disperse seeds. Birds are often important to island 
ecology, since they can easily reach islands. In New Zealand, the Kereru and Kokako are important browsers 
(animals that eat or nibble on leaves, tender young shoots, or other vegetation) and seabirds enrich the soil 
and water with their production of guano (Figure 9, 10). 




Figure 9: The Kereru is an important browser species in New Zealand. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Kereru_maunga.jpg, License: Public Domain) 



331 




Figure 10: The Kokako, another important browser species of New Zealand. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:K0kak0.jpg; License: CC-BY 2.0) 

Finally, let's not forget that birds have had a relationship with humans since the dawn of humanity. Sometimes, 
as in the cooperative honey-gathering among honeyguides and African peoples such as the Borana, these 
relationships are mutualistic, where both benefit. Birds also play prominent and diverse roles in folklore, re- 
ligion, and popular culture, and have been featured in art since prehistoric times, as in early cave paintings. 
Perhaps their beauty and diversity will always capture the imagination of humans. 

Lesson Summary 

• Most of birds' traits are related to their being warm-blooded or their adaptations for flight. 

• Adaptations for flight involve features that are lightweight, flexible, strong and that take advantage of air 
currents. 

• The components of reproduction usually involve a courtship display, nest production, egg-laying, incubation 
and parental care. There is much diversity demonstrated in adaptations for predator avoidance. 

• With 1 0,000 bird species there is a lot of diversity. Specialized structures are adapted for specific habitats 
or living requirements. 

• Birds are important economically, ecologically and in human culture. 
Review Questions 

1 . List five traits which are important for flight. (Beginning) 



332 



2. Describe how a bird's breeding system can be adapted to avoid predation. (Beginning) 

3. Explain how the absence of land predators on islands would result in flightlessness in birds. (Challenging) 

4. You detect the presence of antibodies to the West Nile Virus in young chickens. How did the chickens 
get the virus? When would the first human cases of the virus most likely occur? (Challenging) 

Further Reading I Supplemental Links 

Department of Health, Florida. Available on the web at: www.doh.state.fl.us. 

Oliver L. Austin, Birds of the World. Western Publishing Company, Inc., New York, 1961. 

Unabridged Dictionary, Second Edition. Random House, New York, 1998. 

http://en.wikipedia.org/wiki/West_Nile_virus 

http://www.birds.cornell.edu/AIIAboutBirds/studying 

http://kids.nationalgeographic.com/Animals 

http://www.ucmp.berkeley.edu/diapsids/birds/birdintro.html 

http://www.personal.psu.edU/users/h/j/hjs130/aves.html 

http://www.fs.fed.us/global/wings/birds.htm 

Vocabulary 

aerofoil A surface which is designed to aid in lifting or controlling by making use of the air currents 

through which it moves. 

altricial A reproductive system in birds in which the newly hatched young are small, naked, immobile 

and blind. 

monogamous A mating system in birds where the couple pair for the duration of the breeding season or 
sometimes for a few years or until one mate dies. 

polygamous A mating system in birds where there is more than one mate. 

precocial A reproductive system in birds in which the newly hatched young are feathered and mobile. 

Review Answers 

1. 

a lightweight skeleton with air-filled cavities 

flexible neck vertebrae 

attachment of two large flight muscles to the sternum 

feathers 

modified forelimb functioning as an aerofoil (wing) 

2. Eggs can be hidden in a hole or burrow; if eggs are in the open they may have a camouflaged shell, the 
birds may have camouflaged feathers, or, as in a colonial-nesting bird, the adults may be more capable of 
defending their young. Precocial young do not generally need the protection of nests, since they are feathered 
and mobile after hatching. Ancient murrelet chicks follow their parents out to sea shortly after hatching in 
order to avoid land predators. 



333 



3. If there were no land predators, then birds would not need to fly to escape predators. Therefore, through 
natural selection, flightless birds would be selected over birds with flight. 

4. From mosquitoes biting them. Within three months. 

Points to Consider 

• Birds and mammals are the only warm-blooded vertebrates. As in birds, mammals also have lots of di- 
versity and live in varied habitats. Based on what you know about adaptations in birds, how do you think 
mammalian limbs are adapted for locomotion in different habitats? 

• Mammals also have specialized diets, as in birds. Instead of beaks, mammals have different kinds of 
teeth. How do you think different kinds of teeth in mammals are adapted for different kinds of diets in 
this group? 



Mammals 

Lesson Objectives 

• List and describe general traits of mammals. 

• Compare reproduction in monotremes, marsupials and placental mammals. 

• Describe how mammals can be grouped according to their anatomy and their habitats. 

• Explain how non-human mammals can benefit people and how they play an ecological role. 

Check Your Understanding 

1 . What traits are there in mammals as a result of them being warm-blooded? 

Answer: They have fur to decrease heat loss; their diets contain high energy foods and methods of feeding 
help to maintain a high metabolism; and they conserve energy both by being inactive at certain times of day 
and sometimes by hibernation. 

2. Describe courtship displays in birds. As you learn about mammals, think about how their courtship is 
similar or different to that of birds. 

Answer: Males usually perform courtship displays in birds. Most displays involve a type of song and some 
displays are very elaborate and may include dancing, aerial flights, or wing or tail drumming. 

introduction 

What's a mammal? It is easy to forget about the biodiversity of mammals, but these animals range from 
bats and cats and rats to dogs and monkeys and whales. They walk and run and swim and fly. They live in 
the ocean, they fly in the sky, they walk on the prairies and run in the savannah. What allows them to live 
in such diverse environments? Well, mammals have some specialized traits which no other group of animals 
has. There is a tremendous amount of diversity within the group in terms of reproduction, habitat, and 
adaptation for living in their different habitats. It is because of some of their traits that mammals have been 
of benefit to people and also play an important ecological role. 

Characteristics of Mammals 

Mammals (class Mammalia) are warm-blooded, vertebrate animals with a number of unique characteristics. 
In most mammals, these include: 

• The presence of hair 
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Sweat glands 

Glands specialized to produce milk (mammary glands) 

Three middle ear bones 

A neocortex region in the brain 

Specialized teeth 

A four-chambered heart 

All mammals, except for the monotremes (the most primitive order of mammals, with certain birdlike and 
reptilian characteristics, such as laying eggs, and a single opening for the urinary, genital, and digestive 
organs), produce live young (known as vivipary) instead of laying eggs. 

There are approximately 5,400 mammalian species, ranging in size from the tiny 1 -2 in (30-40mm) bumblebee 
bat to the 1 ,083ft (330m) blue whale. These are distributed in about 1 ,200 genera, 1 53 families and 29 orders, 
(see http://users.tamuk.edu/kfjab02/Biology/mammalogy/mammal_classification.htm). 

Reproduction in Mammals 

Keep in mind what you have learned about reptiles and birds and see how mammals might be both similar 
and different to these groups. The egg-laying monotremes, such as echidnas (Figure 1) and platypuses 
(Figure 2), use one opening, the cloacae, to urinate, deficate and reproduce, just as lizards and birds do. 
They lay leathery eggs, similar to those of lizards, turtles and crocodilians. Monotremes feed their young by 
"sweating" milk from patches on their bellies, since they lack nipples, unlike other mammals. 




Figure 1 : The echidna is a member of the monotremes, the most primitive order of mammals. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Echidna_-_melbourne_zoo.jpg, License: GNU Free 
Documentation) 



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Figure 2: Another monotreme, the platypus, like other mammals in this order, lays eggs and has a single 
opening for the urinary, genital, and digestive organs. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Platypus.jpg, License: GNU Free Documentation) 

All other mammals give birth to live young and are either marsupial or placental. The females of most mar- 
supials have an abdominal pouch or skin fold within which are mammary glands and a place for raising the 
young (Figure 3). Placental mammals have a placenta that nourishes the fetus and removes waste products. 




Figure 3: A marsupial mammal, this Eastern grey kangaroo has a joey (young kangaroo) in its abdominal 
pouch. 

(Source: http://commons.wikimedia.Org/lmage:Kangaroo_andJoey05.jpg, License: GNU-FDL) 

Some mammals are solitary except for brief periods when the female comes into estrus, the optimal time 
for a female to get pregnant. Others form social groups where a pronounced difference between sexes 
(sexual dimorphism) is frequently extreme. Dominant males are often those that are largest or best-armed. 
These males usually have an advantage in mating or may exclude other males from access to females 
within a group, such as in elephant seals (Figure 4). This group of females forms a harem. Think back to 
what you learned about courtship displays in birds. How are such systems in mammals similar or different? 



336 




Figure 4: A mating system with a harem of many females and one male, as seen in the seal species, Cal- 
lorhinus ursinus. 



(Source: http://commons.wikimedia.Org/wiki/lmage:Callorhinus_ursinus_and_harem.jpg, 
Domain) 



License: Public 



Groups of Mammals 

Mammal groups, as is true for most animal groups, can be characterized a number of ways. They can be 
characterized according to their anatomy, the habitats in which they live, and their feeding habits. 

Most mammals belong to the placental group. Within this group are several subgroups including lagomorphs 
(i.e. hares and rabbits) and rodents (rats, mice and other small, gnawing mammals); carnivores (cats, dogs, 
bears and other mammals that are primarily meat eaters) (Figure 5); insectivores (including moles and 
shrews) (Figure 6); a group including bats and primates; and ungulates (hoofed animals, including deer, 
sheep, goats, buffalo and elephants, and also whales and manatees) (Figure 7). 




Figure 5: A Caracal, hunting in the Serengeti. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Caracal_hunting_in_the_serengeti.jpg, License: GNU- 
FDL) 



337 




Figure 6: One of the subgroups of placental mammals is the insectivores, including moles and shrews. 
Pictured here is the Northern short-tailed shrew. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Blarina_brevicauda.jpg, License: CC-BY 2.0) 




Figure 7: The ungulates (hoofed animals) like the giraffe here, is another of the subgroups belonging to the 
placental mammals. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Giraf.jpg, License: GNU-FDL) 

Why do you think the above groups of animals are placed together? Can you think of some examples of 
tooth type that are adapted for a mammals' diet and types of limbs that are adapted for living in different 
types of habitats? 

Mammals can also be grouped according to the habitat they live in and with adaptations for living in that 
habitat. Terrestrial mammals with saltatory (leaping) locomotion, as in some marsupials and in lagomorphs, 
is typically found in mammals living in open habitats. Other terrestrial mammals are adapted for running, 
such as dogs or horses. Still others, such as elephants, hippopotamuses and rhinoceroses, have a cumber- 
some (and hefty) mode of locomotion known as "graviportal." 



338 



Other mammals are adapted for living in trees (arboreal), such as many New World monkeys. Others are 
fully aquatic, such as manatees, whales, dolphins and seals, and others are adapted for flight, as are bats, 
or gliding (some marsupials and rodents). 




Figure 8: This howler monkey shows adaptations for an arboreal existence. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Bugio-006_0003.jpg, License: CC-BY-SA2.0 Brazil) 

Significance of Mammals 

Mammals are thought to be significant both in terms of how they benefit people and also of their importance 
ecologically. Given what you know about mammals so far, how do you think they may be important to people? 
Just examining our daily lives we see examples of mammals (other than people!) serving our needs every- 
where. We have pets that are mammals, most commonly dogs and cats; if we live in rural areas or visit another 
country we will probably see lots of examples of mammals used for transport (horses, donkeys, mules and 
even camels), being raised for food (cows and goats), and used for work (dogs, horses, and elephants). 




Figure 9: A Labrador Retriever working as an assistance dog. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Labrador_Retriever_assistance_dog.jpg) 

The special capabilities of some mammal species have been used in practical situations and also to increase 
our knowledge. Can you think of how they have been used? For example, the United State and Russian 
militaries have trained and employed oceanic dolphins to rescue lost divers or to locate underwater mines. 
Mammals' more highly developed brain has made them ideal for use by scientists in studying such things 
as learning, as seen in maze studies of mice and rats. The ability of young mammals to learn from the ex- 



339 



perience of their elders has allowed a behavioral plasticity unknown in any other group of organisms and 
has been a primary reason for the evolutionary success of mammals. See if you can come up with some 
other examples. 

Mammals have also played a significant role in different cultures' folklore and religion. For example, the 
grace and power of the cougar have been widely admired in the cultures of the indigenous peoples of the 
Americas. The Inca city of Cusco is reported to have been designed in the shape of a cougar and the sky 
and thunder god of the Inca, Viracocha, has been associated with the animal. In North America, mythological 
descriptions of the cougar have appeared in stories of a number of Native American tribes. 

Ecologically, nectar-feeding and fruit-eating bats play an important role in plant pollination and seed dispersal, 
respectively. Can you think of a type of bird that has a similar ecological role? 




Figure 10: Bats, like this Egyptian fruit bat, belong to another subgroup of placental mammals. Ecologically, 
fruit bats play an important role in seed dispersal. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Egyptian.fruitbat.arp.jpg, Photographer: Adrian Pingstone, 
License: Public Domain) 

Mammals are also the only animal group that has made a complete transition to aquatic habitats. Some, 
such as cetaceans (whales, dolphins and porpoises) have undergone profound adaptations for swimming 
and life, even reproduction, in the water. Cetaceans depend on water for mechanical support and thermal 
insulation. Because they are buoyed by their aquatic environment, whales have evolved into the largest 
mammals and the largest animals ever recorded. 

Micro-Lab: Matching Adaptations of Teeth and Limbs in Mammals with their respec- 
tive Diets and Habitats 

Estimated time: 15 minutes 

Materials: 

1 . Tray of actual, or illustrations of, various mammal teeth, numbered, and Pictures of animals eating: 

• Incisors - cutting and nipping (herbivores, like cows, have well-developed incisors for cutting grass) 

• Premolars - shearing and grinding (herbivores, like cows, have flat premolars and molars for grinding 
vegetation) 

• Canines - piercing (carnivores, like lions, have long and strong canines.) 



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2. Tray of actual, or illustrations of, various mammalian limbs, numbered (for feet, could also show cast of 
track, to see if students can match the track with the actual foot type) and pictures of habitats or actual animals, 
lettered: 

Toe ending in claws - tiger (climbing and running) 

Toes with hooves - horses and cows (running) 

Fins - aquatic mammals (swimming) 

Wings- bats (flying) 

Highly mobile limbs - monkeys (climbing in trees) 

3. Answer sheets, listing numbered mammal teeth and limbs 

Directions: 

One group of students examines the tray of mammal teeth and pictures of diets and indicates on the answer 
sheet the correct matches. The other group of students examines the tray of mammal limbs and pictures of 
habitats and similarly matches these up with the correct answers. 

Links to websites with pictures of mammal teeth and/or limbs: Teeth: 

http://www.vinsweb.org/education/elf/units/tas.html 

http://www.teachersdomain.org/resources/tdc02/sci/life/stru/jaws/index.html 

Teeth and Limbs: 

http://www.acornnaturalists.com/Mammal-Activities-C227.aspx 

Lesson Summary 

• The class Mammalia is distinguished by the presence of hair, sweat glands, three middle ear bones and 
a neocortex area in the brain. 

• There is a lot of variation in mammalian reproductive systems. Mammals consist of both the egg-laying 
monotremes and those that are viviparous. The latter group includes marsupial and placental mammals. 
Diversity can also be found in mammalian mating systems. 

• The 5,400 species of mammals can be grouped according to anatomical features as well as the type of 
habitat found in. Mammals have specific adaptations for living on land, in trees, in water and for flight. 

• Non-primate mammals have an important relationship with people as well as fulfilling necessary ecolog- 
ical functions. 

Review Questions 

1. What are two ways in which monotremes differ from viviparous mammals? (Beginning) 

2. With respect to characteristics of feet, limbs and tails, what features would you expect mammals to have 
for: (Intermediate) 

a. jumping? 

b. living in trees? 

3. Give examples of three different adaptations of limbs for locomotion in mammals, naming a mammal 
species, a structure and how it is adapted. (Intermediate) 



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4. Instead of beaks, as in birds, mammals have different kinds of teeth. Incisors are specialized for cutting 
and nipping, premolars for shearing and grinding, and canines for piercing. Based on what you know of diets 
in mammals, name two mammal species, the kind of diet they eat, and one type of specialized teeth that 
would be best adapted for the diet. (Intermediate) 

5. In order to maintain a high constant body temperature, mammals need a nutritious and plentiful diet. What 
are some ways that mammals have adapted to meet their dietary requirements? How might size determine 
diet type, and why? (Intermediate) 

Further Reading I Supplemental Links 

Unabridged Dictionary, Second Edition. Random House, New York, 1998. 

http://en.wikipedia.org 

http://kids.nationalgeographic.com/Animals 

http://kids.yahoo.com/animals/mammals 

http://nationalzoo.si.edu/Animals/SmallMammals/ForKids 

http://www.ucmp.berkeley.edu/mammal/mammal.html 

http://www.americazoo.com/goto/index/mammals/classification.htm 

Vocabulary 

estrus A period of time when the female has maximum sexual receptivity. 

harem A group of females followed or accompanied by a fertile male; this male excludes other 

males access to the group. 

mammary glands Specialized sweat glands that produce milk. 

marsupial A type of mammal where the female has an abdominal pouch or skin fold within which 

are mammary glands and a place for raising the young. 

monotremes A group of mammals that lays eggs and feeds their young by "sweating" milk from 

patches on their bellies. 

neocortex Site of the cerebral cortex where most of higher brain functions occur. 

placental A type of mammal that has a placenta that nourishes the fetus and removes waste 

products. 

vivipary A reproductive system in most mammals and some reptiles and fish, in which living 

young are produced rather than eggs laid. 

Review Answers 

1. Monotremes lay leathery and uncalcified eggs, whereas viviparous mammals give birth to live young. 
Monotremes feed their young by "sweating" milk from patches on their bellies, while viviparous mammals 
secrete milk from their nipples. 

2a. Jumping mammals typically have elongate, plantigrade (with the digits, bones of the midfoot, and parts 
of the ankle and wrist in contact with the ground) hind feet, reduced forelimbs, and long tails. 

2b. Well-adapted arboreal mammals are frequently plantigrade, five-toed, and equipped with highly mobile 
limbs. Many arboreal mammals also have claws or well-developed nails. 

3. A mammal such as the tiger has toes ending in claws. Claws can be used for climbing and running. Toes 
with hooves, which can be used for running, are found in such animals as horses and cows. Aquatic mammals, 
like whales and dolphins, have fins for swimming. Other examples include highly mobile limbs in monkeys 



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for living in trees and wings in bats. 

4. A cow, which feeds on vegetation, would have well-developed incisors for cutting grass and broad, flat 
premolars and molars for grinding the vegetation. Birds and mammals are the only warm-blooded vertebrates. 
As in birds, mammals also have lots of diversity. Instead of beak variation, as in birds, mammals have widely 
varying teeth for different diets. And differences in mammalian feet and claws reflect where and how they 
live. 

5. Mammals can either be carnivorous (eating animal prey, including insectivorous diets). Animals which 
are herbivorous (plant-eating), include those that eat fruit and grasses. Omnivores eat both animal and plant 
material. Since, small mammals have a high ratio of heat-losing surface area to heat-generating volume, 
they tend to have high-energy requirements and a high metabolic rate. Therefore, they are mostly insectiv- 
orous because they cannot tolerate the slow, complex digestive system of a herbivore. Larger animals 
generate more heat, less of which is lost to the outside. They can therefore expend more energy, for example, 
preying on larger vertebrates or tolerate a slower digestive process (herbivores). 

Points to Consider 

• Rats are considered to be highly intelligent as they can learn and perform new tasks, an ability that may 
be important when they first colonize a fresh habitat. Think about what kind of increased learning takes 
place with an increased brain size, as we will see in primates. 

• Think of some significant similarities between the mammals you read about in this lesson with those in 
the next lesson, particularly human beings. 

• What are some significant adaptations in the evolution of primates? 



Primates and Humans 

Lesson Objectives 

• List and describe general traits of primates. 

• Summarize mating systems of primates. 

• Review the types of habitats primates can be found in. 

• Describe the three main groupings of primates. 

• List the traits of the hominids, their diet, reproduction and social system. 

Check Your Understanding 

1 . What are general traits of mammals? 

2. Describe the mating systems in mammals. 

Introduction 

If primates are mammals, what makes them seem so different? Primates, including humans, have several 
unique features only belonging to this group of mammals. Some of these adaptations are obvious, others 
not so obvious. Some of these features give primates advantages such that allow them to live in certain 
habitats, such as arboreal habitats, such as trees. Other features have allowed them to adapt to complex 



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and new social and cultural situations. 

What are Primates? 

The biological order Primates, mostly omnivorous (eating both plant and animal material) mammals, contains 
all the species commonly related to the lemurs, monkeys and apes, the latter including humans. All primates 
have five fingers (pentadactyl), a generalized dental pattern, a primitive (unspecialized) body plan and 
certain eye orbit characteristics, such as a postorbital bar (a bone, which runs around the eye socket). While 
an opposable thumb (the only digit on the hand able to turn back against the other four fingers, thereby re- 
fining the grip in order to hold objects) are a characteristic feature of this group, other orders, such as 
opossums, also have this feature. 




Figure 1 : A ring tailed lemur and twins. Lemurs belong to the prosimian group of primates. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Ring_tailed_lemur_and_twins.jpg, Photographer: Sannse, 
License: GNU-FDL) 




Figure 2: One of the New World monkeys, a squirrel monkey. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Squirrel_monkey_2.jpg, Photographer: Linda de Voider, 
License: CCA-BY-SA 2.5, 2.0, 1 .0) 



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Figure 3: Chimpanzees, pictured here, belong to the great apes, one of the groups of primates. 
(Source: http://commons.wikimedia.Org/wiki/lmage:Chimps.jpg, License: CCA 2.5) 




Figure 4: Reconstruction of a Neanderthal man, belonging to an extinct subspecies of Homo sapiens, humans, 
who are part of the great apes. This subspecies lived in Europe and western and central Asia from about 
100,000 -40,000 B.C. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Neandertaler_rec0nst.jpg, Photographer: Stefan Scheer, 
License: CCA 2.5) 

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Indications 
of intelligence in primates include the ability to learn and complex behavioral flexibility, involving much social 
interaction, such as fighting and play. 

Old World species (apes and some monkeys) tend to have significant sexual dimorphism, characterized 
mostly as size differences, with males being slightly more than twice as heavy as females. This dimorphism 
may be a result of a polygymous mating system where males attract and defend multiple females. New 
World species (including tamarins and marmosets) form pair bonds, which is a partnership between a mating 
pair that lasts at least one season. The pair cooperatively raise the young, and thus generally do not show 
significant size difference between the sexes. 



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AV-; 



Figure 5: An Old World monkey, a species of macaque, in Malaysia. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Langschwanzmakakjn_ti0man.jpg, Photographer: Luxo, 
License: GNU-FDL) 




Figure 6: A New World species of monkey, a tamarin. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Springtamarin.jpg, License: CCA-BY-SA 2.0 Germany) 




Figure 7: Another New World species of monkey, the common marmoset. 



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(Source: http://commons.wikimedia.org/wiki/lmage:WeiBbuschelaffe_(CallithrixJacchus).jpg, Photographer: 
Raimond Spekking, License: GNU-FDL) 

Non-human primates occur mostly in Central and South America, Africa and South Asia. Since primates 
evolved from arboreal animals, many modern species live mostly in trees. Other species are partially terrestrial, 
such as baboons and the Patas monkey. Only a few species are fully terrestrial, for example, the gelada 
and humans. 




Figure 8: Baboons are partially terrestrial. Pictured here is a mother baboon and her young, in Tanzania. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Baboons_on_rock.jpg, Photographer: Charles J. Sharp, 
License: CCA 2.5) 

Primates live in a diverse number of forested habitats, including rain forests, mangrove forests and mountain 
forests to altitudes of over 9,800 ft (3,000 m). The combination of opposable thumbs, short fingernails and 
long, inward-closing fingers has, in part, allowed some species to develop brachiation, locomotion of 
swinging by arms from one branch to another. Another feature for climbing -expanded digits -as in tarsiers 
improves grasping. 




Figure 9: A gibbon shows how its limbs are modified for hanging from trees. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:VVeisshandgibbon_tierpark_berlin.jpg, License: GNU- 
FDL) 



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Figure 10: A species of tarsier, with expanded digits used for grasping branches. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:koboldmaki-drawing.jpg, License: Public Domain) 

A few species, such as the proboscis monkey, De Brazza's monkey and Allen's swamp monkey, the latter 
having small webbing between its fingers, are fine swimmers and occur in swamps and other aquatic habitats. 
Some species, such as the rhesus macaque and the Hanuman langur, can exploit human-altered environ- 
ments and even live in cities. 

Primate Classification 

The primate order is divided informally into three main groupings: prosimians, New World monkeys, and 
Old World monkeys and the apes. The prosimians are species whose bodies most closely resemble that of 
the early proto-primates, the earliest examples of primates. Prosimians include the lemurs, located in 
Madagascar and to a lesser extent on the Comoro Islands, a group of islands in the Indian Ocean. 




Figure 11 : One of the prosimians, a greater bush baby, Kenya. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Greater_Bush_Baby.jpg, Photographer: Buecherfresser, 
License: GNU-FDL) 

The New World monkeys include the capuchin, howler and squirrel monkeys, who live exclusively in the 
Americas. The Old World monkeys and the apes (all except for humans, who inhabit the entire earth) inhabit 
Africa and southern and central Asia. 



348 



A few new species of primates are discovered each year and the evaluation of current populations varies 
as to the number of species; estimates over the last several years range from 350 to 405 species. In New 
World monkeys alone there are thought to be 128 species; of Old World monkeys, 135 species; of gibbons 
or "lesser apes," 13 species and of humans and other great apes, seven species. But there is only one 
species of humans, which will be discussed below. 

The Human Family 

The great apes are the members of the biological family Hominidae, which includes seven species, making 
up humans, two species each of chimpanzees, gorillas and orangutans. Hominids are large, tailless primates, 
ranging in size from the pygmy chimpanzee, at 66-88 lbs (30-40 kg) in weight, to the gorilla, at 309-397 lbs 
(140-180 kg). In all species, the males are, on average, larger and stronger than the females, although the 
degree of sexual dimorphism varies greatly. Most living species are predominantly quadrupedal (four- 
footed), but all are able to use their hands for gathering food or nesting materials, and in some cases, for 
using tools, such as gorillas using sticks to gauge the depth of water and chimpanzees sharpening sticks 
to use as spears in hunting and using sticks to gather food and to "fish" for termites. 




Figure 12: A gorilla mother and baby, members of the great apes, at Volcans National Park, Rwanda. The 
gorilla is the largest of the hominids, getting up to 309-397 lbs. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Gorilla_mother_and_baby_at_Volcans_National_Park.jpg, 
Photographer: Sarel Kramer, License: CCA-BY-SA 2.0) 




Figure 13: Tool using in a primate. A chimpanzee uses a stick to "fish" for termites, and then, pictured here, 
extracts the insects. 

(Source: http://commons.wikimedia.Org/wiki/lmage:BonoboFishing05.jpg, Photographer: Mike R., License: 
CCA-BY-SA 2.5, 2.0, 1.0) 



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Most species are omnivorous (eat both plants and meat), but fruit is the preferred food among all but humans. 
In contrast, humans consume a large proportion of highly processed, low fiber foods, unusual proportions 
of grains and vertebrate meat, as well as a wide variety of other foodstuffs. Human teeth and jaws are 
markedly smaller for our size than those of other apes, perhaps as adaptations to eating cooked food. Humans 
may have been eating cooked food for possibly as long as a million years or more. 

Gestation lasts 8-9 months and usually results in the birth of a single offspring. The young are born helpless, 
and thus they need parental care for long periods of time. Compared with most other mammals, great apes 
have a long adolescence and become fully mature not until 8-13 years in most species (longer in humans). 
Thus, females typically give birth only once every few years. 

Gorillas and chimpanzees live in family groups of approximately five to ten individuals, although larger groups 
are sometimes observed. The groups include at least one dominant male, and females leave the group at 
maturity. Orangutans, however, are generally solitary. Human social structure is complex and highly variable. 
Can you think of any that are similar to those of other great apes? 

Gorillas, chimpanzees and humans are all lumped together in the subfamily, the Homininae, because they 
generally share more than 97% of their DNA with each other, and exhibit a capacity for language or for 
simple culture beyond the family or band, a group of animals functioning together. A proposed theory including 
such faculties as empathy is a controversial criterion distinguishing the adult human alone among the ho- 
minids. Can you think of other human attributes that are unique to humans? 

Lesson Summary 

• Primates are characterized by pentadactyly, a generalized dental pattern, an unspecialized body plan 
and certain eye orbit features. Primates also have opposable thumbs and a large cerebrum relative to 
the rest of the brain. 

• Old World species tend to have significant sexual dimorphism, whereas New World species generally 
do not show significant sexual differences. 

• Many primates live in a variety of forested habitats, whereas others are partially terrestrial, and some, 
like the gelada and humans, are fully terrestrial. A few species are adapted for living in aquatic habitats. 

• There are three subgroups within the primates order: prosimians, including the lemurs; New World 
monkeys, and the Old World monkeys and the apes. There are estimated to be somewhere between 
350 to 405 species of primates. 

• The great apes, consisting of seven species, are large, tailless primates, with sexual dimorphism. Most 
species are quadrupedal, but all are able to use their hands. 

• Most great apes are omnivorous, but fruit is the preferred food among all species but humans. 

• The great apes have unique reproductive and parental care features, especially when compared with 
most other mammals. There is a variety of social structure among the great apes. 

• Gorillas, chimpanzees and humans share some common characteristics. 
Review Questions 

1. What characteristics distinguish the biological order Primates? (Beginning) 

2. What theory might explain why human teeth and jaws are markedly smaller for our size than those of 
other apes? (Beginning) 

3. Opposable thumbs are a characteristic primate feature. List two ways in which non-human primates might 
use opposable thumbs. (Intermediate) 

4. Various hybrid monkeys are produced in captivity when different species or subspecies are housed together. 
In what situation in the wild would hybrids be produced? (Intermediate) 



350 



5. Primates are thought to have developed several of their traits and habits initially while living in trees. What 
primate features might be an advantage in an arboreal habitat? (Intermediate) 

6. Gorillas and chimpanzees live in family groups of around five to 10 individuals. What are two possible 
strategies for feeding, when fruit is hard to find? (Intermediate) 

Further Reading I Supplemental Links 

Unabridged Dictionary, Second Edition. Random House, New York, 1998. 

http://kids.nationalgeographic.com/Animals 

http://nationalzoo.si.edu/Animals/Primates 

http://www.ucmp.berkeley.edu/mammal/eutheria/primates.html 

http://pslc.ws/macrog/paul/lemurs.htm 

http://www.wikipedia.org 

Vocabulary 

hybrid The offspring of different species, genera, varieties or breeds. 

omnivorous Eating both plant and animal material. 

pentadactyl Having five fingers or toes. 

quadrupedal four-footed 

sexual dimorphism A condition in which the males and females of a species are different in form and 
structure. 

Review Answers 

1. All primates have five fingers, a generalized dental pattern, an unspecialized body plan and certain eye 
orbit characteristics. 

2. The relative size of human teeth and jaws may be an adaptation for eating cooked food for as long as a 
million years or more. 

3. Opposable thumbs allow some species to use tools to perform some tasks. The use of opposable thumbs 
allows some species to develop brachiation. 

4. Where the ranges of two species or subspecies overlap, then you can get hybrids produced. 

5. a. Primates relied on sight over the sense of smell. They were able to develop a keen sense of depth 
perception which can be attributed to the constant leaping that was necessary to move about the trees. 

b. Primates also developed hands and feet that were capable of grasping, a requirement for crawling along 
branches and for reaching out for various types of food growing in the trees they inhabited. Claws, well-de- 
veloped nails and expanded pads on the digits improve grasping. 

c. Some species, including many New World monkeys, have a prehensile tail, which is used like a fifth hand. 

6. a. Eat other parts of plants, like leaves and shoots (gorillas do this). 



351 



b. Split up into smaller groups to find what fruit might be available (a strategy used by chimpanzees). 

Points to Consider 

• Forward-facing color binocular vision was useful for human ancestors who swung by their arms from 
one branch to another. Recent studies suggest this type of vision was more useful in courtship. What 
other groups of animals might vision also be important in courtship? 

• Thousands of primates are used every year around the world in scientific experiments because of their 
psychological and physiological similarity to humans. What kinds of behavioral experiments do you think 
might be conducted in primates? 



352 



15. Behavior of Animals 



Understanding Animal Behavior 

Lesson Objectives 

• Give examples of animal behavior. 

• Explain why animal behavior is important. 

• Describe innate behavior and how it evolves. 

• List ways that behavior can be learned. 

Do you have a dog or a cat? If you don't, you probably know someone that does. Think about how these 
animals act. Does the dog bark when it's excited? Does the cat purr when it's happy? Do they both play 
with toys? 

Examples of Animal Behavior 

Barking, purring, and playing are just some of the ways that dogs and cats behave. These are examples of 
animal behavior. Animal behavior is any way that animals act, either alone or with other animals. Can you 
think of other examples of animal behavior? What about insects and birds? How do they behave? The pictures 
in Figure 1 show some of the ways that these and other animals act. Look at the pictures and read about 
the behaviors. 

Figure 1 (a-g): All of the animals pictured here are busy doing something important. Read about what each 
animal is doing. Think about why the animal is behaving that way. These are just a few of the many ways 
that animals behave. 



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1 




Figure 1a: This cat is stalking a mouse. It is a hunter by nature. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/9/94/Hunting_cat.jpg, License: Creative Commons 
Attribution 1.0) 




Figure 1b: This spider is busy spinning a web. If you have ever walked into a spider web, you know how 
sticky a spider web can be. Why do spiders spin webs? 

(Source: http://commons.wikimedia.Org/wiki/lmage:Pajeczyna2.jpg, License: GFDL) 



354 




Figure 1c: This mother dogs is nursing her puppies. In what other ways do mother dogs care for their pup- 
pies? 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Breeding.jpg, License: GFDL) 




Figure 1d: This bird is using its beak to add more grass to its nest. What will the bird use its nest for? 
(Source: http://commons.wikimedia.0rg/wiki/lmage:Nest_bird.jpg, License: Public Domain) 




Figure 1e: This wasp is starting to build a nest. Have you seen nests like this on buildings where you live? 
Why do wasps build nests? 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Wasp_starting_nest.jpg, License: GFDL) 



355 




Figure 1f: This rabbit is running away from a fox. Did you ever see a rabbit run? Do you think you could run 
that fast? 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Wild_rabbit.jpg, License: GFDL) 




Figure 1g: This lizard is perched on a rock in the sun. Lizards like to lie on rocks and "sun" themselves. Do 
you know why? 

(Source: http://commons.wikimedia.0rg/wiki/lmage:LagartoDestaque.jpg, License: CC-BY-SA 2.0) 

Importance of Animal Behavior 

Why do animals behave in these ways? The answer to this question depends on what the behavior is. A 
cat chases a mouse to catch it. A spider spins its sticky web to trap insects. A mother dog nurses her puppies 
to feed them. All of these behaviors have the same purpose: getting or providing food. All animals need food 
for energy. They need energy to move around. In fact, they need energy just to stay alive. Baby animals 
also need energy to grow and develop. 

Birds and wasps build nests to have a safe place to store their eggs and raise their young. Many other animals 
build nests for the same reason. Animals protect their young in other ways, as well. For example, a mother 
dog not only nurses her puppies. She also washes them with her tongue and protects them from strange 
people or other animals. All of these behaviors help the young survive and grow up to be adults. 

Rabbits run away from foxes and other predators to stay alive. Their speed is their best defense. Lizards 
sun themselves on rocks to get warm because they cannot produce their own body heat. When they are 
warmer, they can move faster and be more alert. This helps them escape from predators, as well as find 
food. 



356 



All of these animal behaviors are important. They help the animals get food for energy, make sure their 
young survive, or ensure that they survive themselves. Behaviors that help animals or their young survive 
increase the animals' fitness. You read about fitness in Chapter 7. Animals with higher fitness have a better 
chance of passing their genes to the next generation. If behaviors that increase fitness are controlled by 
genes, the behaviors become more common in the species. This is called evolution by natural selection. 

Innate Behavior 

All of the behaviors shown in Figure 1 are ways that animals act naturally. They don't have to learn how to 
behave in these ways. Cats are natural-born hunters. They don't need to learn how to hunt. Spiders spin 
their complex webs without learning how to do it from other spiders. Birds and wasps know how to build 
nests without being taught. Behaviors such as these are called innate. 

An innate behavior is any behavior that occurs naturally in all animals of a given species. An innate behavior 
is also called an instinct. The first time an animal performs an innate behavior, the animal does it well. The 
animal does not have to practice the behavior in order to get it right or become better at it. Innate behaviors 
are also predictable. All members of a species perform an innate behavior in the same way. From the exam- 
ples described above, you can probably tell that innate behaviors usually involve important actions, like 
eating and caring for the young. 

There are many other examples of innate behaviors. For example, did you know that honey bees dance? 
The honey bee in Figure 2 has found a source of food. When the bee returns to its hive, it will do a dance, 
called the waggle dance. The way the bee moves during its dance tells other bees in the hive where to find 
the food. Honey bees can do the waggle dance without learning it from other bees, so it is an innate behavior. 




Figure 2: When this honey bee goes back to its hive, it will do a dance to tell the other bees in the hive 
where it found food. 

(Source: http://en.wikipedia.Org/wiki/lmage:Bee_on_Geraldton_Wax_Flower.jpg, License: Public Domain) 

Besides building nests, birds have other innate behaviors. One example occurs in gulls. Figure 3a shows 
a mother gull and two of her chicks. One of the chicks is pecking at a red spot on the mother's beak. This 
innate behavior causes the mother to feed the chick. In many other species of birds, the chicks open their 
mouths wide whenever the mother returns to the nest. This is what the baby birds in Figure 3b are doing. 
This innate behavior, called gaping, causes the mother to feed them. 



357 




Figure 3a: This mother gull will feed her chick after it pecks at a red spot on her beak. Both pecking and 
feeding behaviors are innate. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Larus_D0minicanus.jpg, License: GFDL) 




Figure 3b: When these baby birds open their mouths wide, the mother instinctively feeds them. This innate 
behavior is called gaping. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Bird_nest.jpg, License: GFDL) 

Another example of innate behavior in birds is egg rolling. It happens in some species of water birds, like 
the graylag goose shown in Figure 4. Graylag geese make nests on the ground. If an egg rolls out of the 
nest, a mother goose uses her bill to push it back into the nest. Returning the egg to the nest helps ensure 
that the egg will hatch. 




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Figure 4: This female graylag goose is a ground-nesting water bird. Behind her are two of her young chicks. 
Before the chicks hatch, the mother protects the eggs. She will use her bill to push eggs back into the nest 
if they rollout. This is an example of an innate behavior. How could this behavior increase the mother goose's 
fitness? 

(Source: http://en.wikipedia.0rg/wiki/lmage:Greylag_Goose_8OO.jpg, License: GDFL) 

Drawback of Innate Behavior 

Innate behaviors such as these usually help animals or their offspring survive. Therefore, they increase fitness. 
This is why the behaviors evolved. However, innate behaviors have a drawback. The trouble with innate 
behaviors is that they are not flexible. An innate behavior is always performed exactly the same way. 

The example of the graylag goose shows how this can be a problem. The sight of any nearby egg-shaped 
object will cause a graylag goose to push the object into her nest. She will push the object even if it isn't an 
egg. For example, if the mother goose sees a golf ball nearby, she will push it into her nest. This wastes 
time and energy that could be spent on the real eggs. From this example, you can see that innate behavior 
is not always helpful. It does not always increase fitness. 

Innate Behavior in Human Beings 

All animals have innate behaviors, even human beings. Can you think of human behaviors that do not have 
to be learned? Chances are, you will have a hard time thinking of any. The only truly innate behaviors in 
humans are called reflex behaviors. They occur mainly in babies. Like innate behaviors in other animals, 
reflex behaviors in human babies may help them survive. 

An example of a reflex behavior in babies is the sucking reflex. Newborns instinctively suck on a nipple that 
is placed in their mouth. It is easy to see how this behavior evolved. It increases the chances of a baby 
feeding and surviving. 

Another example of a reflex behavior in babies is the grasp reflex. This behavior is shown in Figure 5. Babies 
instinctively grasp an object placed in the palm of their hand. Their grip may be surprisingly strong. How do 
you think this behavior might increase a baby's chances of surviving? 




Figure 5: One of the few innate behaviors in human beings is the grasp reflex. It occurs only in babies. 



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(Source: http://en.wikipedia.0rg/wiki/lmage:Greifreflex.jpg, License: Public Domain) 

Learned Behavior 

Just about all other human behaviors are learned and not innate. Learned behavior is behavior that occurs 
only after experience or practice. Learned behavior has an advantage over innate behavior. It is more flexible. 
Learned behavior can be changed if conditions change. For example, you probably know the route from 
your house to your school. Assume that you moved to a new house in a different place, so you had to take 
a different route to school. What if following the old route was an innate behavior? You would not be able 
to adapt. Fortunately, it is a learned behavior. You could learn the new route just as you learned the old one. 

Although most animals can learn, animals with greater intelligence are better at learning and have more 
learned behaviors. Humans are the most intelligent animals. They depend on learned behaviors more than 
any other species. Other highly intelligent species include the apes, our closest relatives in the animal 
kingdom. You read about apes in Chapter 14. They include chimpanzees and gorillas. Both are also very 
good at learning behaviors. 

You may have heard of a gorilla named Koko. Koko was raised by the psychologist Dr. Francine Patterson. 
Dr. Patterson wanted to find out if gorillas could learn human language. Starting when Koko was just one 
year old, Dr. Patterson taught her to use sign language. Koko learned to use and understand more than 
1,000 signs. She is shown in Figure 6 using some of the signs. Koko showed how much gorillas can learn. 

Figure 6: Koko the gorilla was raised by a scientist that taught her to use sign language. These are just a 
few of the signs that Koko learned to use. 

Think about some of the behaviors you have learned. They might include riding a bicycle, using a computer, 
and playing a musical instrument or sport. You probably did not learn all of these behaviors in the same 
way. Perhaps you learned some behaviors on your own, just by practicing. Other behaviors you may learned 
from other people. Humans and other animals can learn behaviors in several different ways. Some common 
ways of learning are habituation, observational learning, conditioning, play, and insight learning. 

Habituation 

Habituation is learning to get used to something after being exposed to it for awhile. Habituation usually 
involves getting used to something that is annoying or frightening but not dangerous. Habituation is one of 
the simplest ways of learning. It occurs in just about every species of animal. 

You have probably learned through habituation many times. For example, maybe you were reading a book 
when someone turned on a television in the same room. At first, the sound of the television may have been 
annoying. After awhile, you may no longer have noticed it. If so, you had become habituated to the sound. 

Figure 7 shows another example of habituation. Crows and most other birds are usually afraid of people. 
They avoid coming close to people, or they fly away when people come near them. The crows landing on 
this scarecrow have gotten used to a "human" in this place. They have learned that the scarecrow poses 
no danger. They are no longer afraid to come close. They have become habituated to the scarecrow. 



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Figure 7: This scarecrow is no longer scary to these crows. They have gotten used to it being in this spot 
and learned that it is not dangerous. This is an example of habituation. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/6/64/Scarecrow1.jpg, License: GFDL) 

Can you see why habituation is useful? It lets animals ignore things that will not harm them. Without habitu- 
ation, animals might waste time and energy trying to escape from things that are not really dangerous. 

Observational Learning 

Observational learning is learning by watching and copying the behavior of someone else. Human children 
learn many behaviors this way. When you were a young child, you may have learned how to tie your shoes 
by watching your dad tie his shoes. More recently, you may have learned how to dance by watching a pop 
star dancing on TV. Most likely you have learned how to do math problems by watching your teachers do 
problems on the board at school. Can you think of other behaviors you have learned by watching and 
copying other people? 

Other animals also learn through observational learning. For example, young wolves learn to be better 
hunters by watching and copying the skills of older wolves in their pack. Another example of observational 
learning is shown in Figure 8. These monkeys have learned how to wash their food in the ocean. They 
learned by watching and copying the behavior of other monkeys. 

Figure 8: Washing their food is not an innate behavior in these monkeys. They learned how to do it by ob- 
serving other members of their troop. 

Conditioning 

Conditioning is a way of learning that involves a reward or punishment. Did you ever train a dog to fetch 
a ball or stick by rewarding it with treats? If you did, you were using conditioning. Another example of condi- 
tioning is shown in Figure 9. This lab rat has been taught to "play basketball" by being rewarded with food 
pellets. Conditioning also occurs in wild animals. For example, bees learn to find nectar in certain types of 
flowers because they have found nectar in those flowers before. 



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Figure 9: This rat has been taught to put the ball through the hoop by being rewarded with food for the be- 
havior. This is an example of conditioning. What do you think would happen if the rat was no longer rewarded 
for the behavior? 

(Source: http://www.cosi.org/files/lmage/photos/press/RatBasketball01 .jpg, License: Used by permission - 
COSI Columbus, COSI.org) 

Humans learn behaviors through conditioning, as well. A young child might learn to put away his toys by 
being rewarded with a bedtime story. An older child might learn to study for tests in school by being rewarded 
with better grades. Can you think of behaviors you learned by being rewarded for them? 

[begin sidebar] 

Did you ever hear the saying, "You can't teach an old dog new tricks"? Don't believe it. Older dogs — like 
older people — are capable of learning new behaviors. They may learn more slowly, but they can still learn 
to behave in new ways. 

[end sidebar] 

Conditioning does not always involve a reward. It can involve a punishment instead. A toddler might be 
punished with a time-out each time he grabs a toy from his baby brother. After several time-outs, he may 
learn to stop taking his brother's toys. A dog might be scolded each time she jumps up on the sofa. After 
repeated scolding, she may learn to stay off the sofa. A bird might become ill after eating a poisonous insect. 
The bird may learn from this "punishment" to avoid eating the same kind of insect in the future. 

Learning by Playing 

Most young mammals — including humans — like to play. Play is one way they learn skills they will need as 
adults. Think about how kittens play. They pounce on toys and chase each other. This helps them learn 
how to be better predators when they are older. Big cats also play. The lion cubs in Figure 10a are playing 
and practicing their hunting skills at the same time. The dogs in Figure 10b are playing tug-of-war with a 
toy. What do you think they are learning by playing together this way? Other young animals play in different 
ways. For example, young deer play by running and kicking up their hooves. This helps them learn how to 
escape from predators. 



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Figure 10a: These two lion cubs are playing. They are not only having fun. They are also learning how to 
be better hunters. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Li0n_cubs_Serengeti.jpg, License: CC-BY-SA 2.0) 




Figure 10b: They are really playing. This play fighting can help them learn how to be better predators. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:D0gs_playing.jpg, License: CC-BY-SA 2.0) 

Human children learn by playing, as well. For example, playing games and sports can help them learn to 
follow rules and work with others. The baby in Figure 11 is playing in the sand. She is learning about the 
world through play. What do you think she might be learning? 




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Figure 11 : Playing in a sandbox is fun for young children. It can also help them learn about the world. For 
example, this child may be learning that sand is soft. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:CrhSand.jpg, License: CC-BY-SA 2.5) 

Insight Learning 

Insight learning is learning from past experiences and reasoning. It usually involves coming up with new 
ways to solve problems. Insight learning generally happens quickly. An animal has a sudden flash of insight. 

Insight learning requires relatively great intelligence. Human beings use insight learning more than any other 
species. They have used their intelligence to solve problems ranging from inventing the wheel to flying 
rockets into space. Think about problems you have solved. Maybe you figured out how to solve a new type 
of math problem or how to get to the next level of a video game. If you relied on your past experiences and 
reasoning to do it, then you were using insight learning. 

One type of insight learning is making tools to solve problems. Scientists used to think that humans were 
the only animals intelligent enough to make tools. In fact, being able to make tools was thought to be one 
of the most important human traits. Tool making was believed to set humans apart from all other animals. 
Then, in 1960, chimpanzee expert Jane Goodall discovered that chimpanzees also make tools. She saw a 
chimpanzee strip leaves from a twig. Then he poked the twig into a hole in a termite mound. After termites 
climbed onto the twig, he pulled the twig out of the hole and ate the insects clinging to it (see Figure 12). 
The chimpanzee had made a tool to "fish" for termites. He had used insight to solve a problem. 

Figure 12: This chimpanzee was the first nonhuman primate ever observed to make tools. He was studied 
by Jane Goodall. He is eating termites from the "fishing pole" he made from a twig. 

Since then, chimpanzees have been seen making several different types of tools. For example, they sharpen 
sticks and use them as spears for hunting. They use stones as hammers to crack open nuts. Scientists have 
also observed other species of animals making tools to solve problems. A crow was seen bending a piece 
of wire into a hook. Then the crow used the hook to pull food out of a tube. An example of a gorilla using a 
walking stick is shown in Figure 1 3. Behaviors such as these show that other species of animals — not just 
humans — can use their experience and reasoning to solve problems. They can learn through insight. 



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Figure 13: This gorilla is using a branch as a tool. She is leaning on it to keep her balance while she reaches 
down into swampy water to catch a fish. 

(Source: http://en.wikipedia.0rg/wiki/lmage:G0rrila_t00l_use-Efi.jpg, License: Creative Commons Attribution 
2.5) 

Lesson Summary 

1 . Animal behavior is any way that animals act, either alone or with other animals. 



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2. Behaviors that increase fitness can evolve through natural selection. 

3. Innate behavior is behavior that occurs naturally in all members of a species. 

4. Learned behavior is behavior that occurs only after experience or practice. 
Review Questions 

Knowledge and Comprehension 

1 . Give two examples of animal behavior. (Beginning) 

2. Define innate behavior. (Beginning) 

3. Identify one drawback of innate behavior. (Intermediate) 

4. What is learned behavior? (Intermediate) 

5. State three ways that behavior can be learned. (Intermediate) 
Critical Thinking 

6. Explain how egg rolling by graylag geese is likely to have evolved. (Challenging) 

7. Describe how the grasp reflex might help a baby survive. (Challenging) 

8. Explain how you could use conditioning to teach a dog to sit. (Intermediate) 

9. Why is play important for baby animals? (Intermediate) 

10. A crow was seen dropping nuts on a rock to crack the shells and then eating the nut meats. No other 
crows in the flock were ever observed cracking nuts in this way. What type of learning could explain the 
behavior of this crow? (Challenging) 

Further Reading I Supplemental Links 

CK-12 Foundation. High School Biology, Chapter 34, "Animal Behavior." 

Melvin Berger. Dogs Bring Newspapers but Cats Bring Mice: and Other Fascinating Facts about Animal 
Behavior. Scholastic, 2004. 

Paolo Casale and Gian Paolo Faescini. Animal Behavior: Instinct, Learning, Cooperation. Barrons Juveniles, 
1999. 

http://asci.uvm.edu/course/asci001/behavior.html 

http://news.bbc.co.Uk/1/hi/sci/tech/2178920.stm 

http://news.nationalgeographic.eom/news/2005/1 0/1 025_051 025_gorillas_tools.html 

http://school.discoveryeducation.com/lessonplans/programs/animalinstincts/ 

http://science.jrank.org/pages/3608/lnstinct-Classic-examples-animal-instinct.html 

http://www.biology-online.org/dictionary/lnsight_learning 

http://www.britannica.com/eb/article-48658/animal-behaviour 

http://www.discoverchimpanzees.org/behaviors/top. php?dir=Tool_Use&topic=Termite_Fishing 

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www.janegoodall.org/ 

http://www.keepkidshealthy.com/newborn/newborn_reflexes.html 

http://www.nature.com/hdy/journal/v82/n4/full/6885270a.html 

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1236726#pbio-0030380-b03 

http://www.unmc.edu/Physiology/Mann/mann19.html 

Vocabulary 

animal behavior Any way that animals act, either alone or with other animals. 

innate behavior Any behavior that occurs naturally in all animals of a given species. 

instinct Another term for an innate behavior. 

reflex behaviors The only truly innate behaviors in humans, occurring mainly in babies. 

learned behavior Behavior that occurs only after experience or practice. 

habituation Learning to get used to something that is not dangerous after being exposed to it for awhile. 

observational learning Learning by watching and copying the behavior of someone else. 

conditioning Way of learning that involves a reward or punishment. 

insight learning Learning from past experiences and reasoning. 

Review Answers 

1 . Sample answer: Two examples of animal behavior are cats chasing mice and spiders spinning webs. 

2. Innate behavior is any behavior that occurs naturally in all animals of a given species. An innate behavior 
is also called an instinct. 

3. One drawback of innate behavior is that it is not flexible. An innate behavior is always performed exactly 
the same way. The behavior cannot be changed if conditions change. 

4. Learned behavior is behavior that occurs only after experience or practice. 

5. Three ways that behavior can be learned are (any three): habituation, observational learning, conditioning, 
play, and insight learning. 

6. If an egg rolls out of her nest, a mother graylag goose uses her bill to push it back into the nest. Returning 
the egg to the nest helps ensure that the egg will hatch. This behavior makes it more likely that the mother 
goose will have offspring. Therefore, it increases herfitness. Animals with higherfitness have a better chance 
of passing their genes on to the next generation. If the behaviors that increase fitness are controlled by 
genes, the behaviors are likely to become more common in the species. This is how egg rolling is likely to 
have evolved. 

7. Babies instinctively grasp an object placed in the palm of their hand. This behavior might help a baby 
survive by letting the baby hang on to its mother or other caretaker if the baby started to fall. 

8. You could use conditioning to teach a dog to sit by rewarding the dog with a treat whenever it sat down 
after you gave the command "sit." Eventually, the dog would learn to sit on command in order to get a treat. 



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9. Play is important for baby animals because it helps them learn skills they will need as adults. For example, 
kittens play by pouncing on toys and chasing each other. This helps them learn to be better predators when 
they are older. 

10. The crow was using a new way to solve a problem. Insight learning involves coming up with new ways 
to solve problems. Other crows have been observed using insight learning to get food. Therefore, insight 
learning could explain the behavior of this crow. The behavior was probably nof learned through observational 
learning, because no other crows in the flock were ever observed performing the behavior. 

Points to Consider 

Did you ever watch a long line of ants marching away from their ant hill? What were they doing? How were 
they able to work together? What explains group behaviors such as this? 



Types of Animal Behavior 

Lesson Objectives 

List ways that animals communicate. 
Describe social behavior in animals. 
Explain the purpose of mating behavior. 
Describe how animals defend their territory. 
Identify animal behaviors that occur in cycles. 

Check Your Understanding 

• What is an animal? 

• Give examples of a wide variety of animals. 

• List some "behaviors" animals, such as spiders and rabbits, have in common. 

Introduction 

What is reproduction? (Reproduction is the production of offspring. Animals reproduce asexually or sexually. 
Reproduction is related to fitness because fitness depends in part on the ability to reproduce. Do all animals 
talk to each other? Probably not, but many do communicate. Like human beings, many other animals live 
together in groups. Some insects, including ants and bees, are well known for living in groups. In order for 
animals to live together in groups, they must be able to communicate with each other. Animal communication, 
like most other animal behaviors, increases fitness. Fitness is the ability to survive and have offspring. 
Communication increases fitness by helping animals find food, defend themselves from predators, mate, 
and care for offspring. 

Communication 

What does the word communication make you think of? Talking on a cell phone? Texting? Writing? Those 
are just a few of the ways that human beings communicate. Most other animals also communicate. Com- 
munication is any way that animals share information, and they do this in many different ways. 

Ways That Animals Communicate 

Some animals communicate with sound. Most birds communicate this way. Birds use different calls to warn 
other birds of danger or to tell them to flock together. Many other animals also use sound to communicate. 



368 



For example, monkeys use warning cries to tell other monkeys in their troop that a predator is near. Frogs 
croak to attract female frogs as mates. Gibbons use calls to tell other gibbons to stay away from their area. 

Another way some animals communicate is with sight. By moving in certain ways or "making faces," they 
show other animals what they mean. Most primates communicate in this way. For example, a male chim- 
panzee may raise his arms and stare at another male chimpanzee. This warns the other chimpanzee to 
keep his distance. The chimpanzee in Figure 1a may look like he is smiling. However, he is really showing 
fear. He is communicating to other chimpanzees that he will not challenge them. Look at the peacock in 
Figure 1b. Why is he raising his beautiful tail feathers? He is also communicating. He is showing females 
of his species that he would be a good mate. 

All of the animals pictured here are busy doing something important. Read about what each animal is doing 
then think about why the animal is behaving that way. These are just a few of the many ways that animals 
behave. 




Figure 1a: This chimpanzee is communicating with his face. His expression is called a "fear grin." It tells 
other chimpanzees that he is not a threat. 

(Source: http://commons.wikimedia.org/wiki/lmage: Young_male_chimp.png, License: Creative Commons) 




Figure 1b: This peacock is using his tail feathers to communicate. What is he "saying"? 

(Source: http://en.wikipedia.org/wiki/lmage: Peacock_front02_-_melbourne_zoo.jpg, License: GNU Free 
Documentation) 



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Some animals communicate with scent. They secrete chemicals that other animals of their species can 
smell or detect in some other way. Ants secrete many different chemicals. Other ants detect the chemicals 
with their antennae. This explains how ants are able to work together. The different chemicals that ants secrete 
have different meanings. Some of the chemicals signal all the ants in a group to come together. Other 
chemicals warn of danger. Still other chemicals mark trails to food sources. When an ant finds food, it marks 
the trail back to the nest by secreting a chemical on the ground. Other ants follow the chemical trail to the 
food. 

Many other animals also use chemicals to communicate. You have probably seen male dogs raise their leg 
to urinate on a fire hydrant or other object. Did you know that the dogs were communicating? They were 
marking their area with a chemical in their urine. Other dogs can smell the chemical. The scent of the 
chemical tells other dogs to stay away. 

Human Communication 

Like other animals, humans communicate with one another. They mainly use sound and sight to share in- 
formation. The most important way that humans communicate is with language. Language is the use of 
symbols to communicate. In human languages, the symbols are words. They stand for many different things. 
Words stand for things, people, actions, feelings, or ideas. Think of several common words. What does each 
word stand for? 

Another important way that humans communicate is with facial expressions. Look at the faces of the young 
children in Figure 2. Can you tell from their faces what the children are feeling? Humans also use gestures 
to communicate. What are people communicating when they shrug their shoulders? When they shake their 
head? These are just a few examples of the ways that humans share information without using words. 




Figure 2a: What does this girl's face say about how she is feeling? 

(Source: http://en.wikipedia.org/wiki/lmage: Asian_girl_with_dimples.jpg, License: Creative Commons Attri- 
bution 2.0) 

Social Behavior 

Why is animal communication important? Without it, animals would not be able to live together in groups. 
Animals that live in groups with other members of their species are called social animals. Social animals 
include many species of insects, birds, and mammals. Specific examples of social animals are ants, bees, 



370 



crows, wolves, and humans. To live together with one another, these animals must be able to share infor- 
mation. 

Highly Social Animals 

Some species of animals are very social. In these species, members of the group depend completely on 
one another. Different animals within the group have different jobs. Therefore, group members must work 
together for the good of all. Most species of ants and bees are highly social animals. 

Ants, like those in Figure 3, live together in large groups called colonies. A colony may have millions of ants. 
All of the ants in the colony work together as a single unit. Each ant has a specific job. Most of the ants are 
workers. Their job is to build and repair the colony's nest. Worker ants also leave the nest to find food for 
themselves and other colony members. The workers care for the young, as well. Other ants in the colony 
are soldiers. They defend the colony against predators. Each colony also has a queen. Her only job is to 
lay eggs. She may lay millions of eggs each month. A few ants in the colony are called drones. They are 
the only male ants in the colony. Their job is to mate with the queen. 




Figure 3: The ants in this picture belong to the same colony. They have left the colony's nest to search for 
food. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:A_Texas_Ant_C0l0ny.jpg, License: Creative Commons) 

Honeybees and bumblebees also live in colonies. A colony of honeybees is shown in Figure 4. Each bee 
in the colony has a particular job. Most of the bees are workers. Young worker bees clean the colony's hive 
and feed the young. Older worker bees build the waxy honey comb or guard the hive. The oldest workers 
leave the hive to find food. Each colony usually has one queen that lays eggs. The colony also has a small 
number of male drones. They mate with the queen. 



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■f '.;■*■ .-• ■ 




Figure 4: All the honeybees in this colony work together. Each bee has a certain job to perform. The bees 
are gathered together to fly to a new home. How do you think they knew it was time to gather together? 

(Source: http://en.wikipedia.org/wiki/lmage: Bee_swarm_on_fallen_tree03.jpg, License: GNU Free Docu- 
mentation) 

Cooperation 

Ants, bees, and other social animals must cooperate. Cooperation means working together with others. 
Members of the group may cooperate by sharing food. They may also cooperate by defending each other. 
Look at the ants in Figure 5. They show clearly why cooperation is important. A single ant would not be able 
to carry this large insect back to the nest to feed the other ants. With cooperation, the job is easy. 




Figure 5: These ants are cooperating. By working together, they are able to move this much larger insect 
prey back to their nest. At the nest, they will share the insect with other ants that do not leave the nest. 

(Source: http://www.wettropics.gov.au/st/rainforest_explorer/Resources/lmages/animals/invertebrates/ 
greenTreeAnt.jpg, License: Public Domain) 

Animals in many other species cooperate. For example, lions live in groups called prides. A lion pride is 
shown in Figure 6. All the lions in the pride cooperate. Male lions work together to defend the other lions in 
the pride. Female lions work together to hunt. Then they share the meat with other pride members. 



372 




Figure 6: Members of this lion pride work together. Males cooperate by defending the pride. Females coop- 
erate by hunting and sharing the food. 

(Source: http://commons.wikimedia.org/wiki/lmage: Pride_of_lions.JPG, License: Public Domain) 

Meerkats are small mammals that live in Africa. They also live in groups and cooperate with one another. 
For example, young female meerkats act as babysitters. They take care of the baby meerkats while their 
parents are away looking for food. 

Mating Behavior 

Some of the most important animal behaviors involve mating. Mating is the pairing of an adult male and 
female to produce young. Adults that are most successful at attracting a mate are most likely to have offspring. 
Traits that help animals attract a mate and have offspring increase their fitness. If the traits are controlled 
by genes, they will become more common in the species through natural selection. 

Courtship Behaviors 

In many species, females choose the male they will mate with. For their part, males try to be chosen as 
mates. They show females that they would be a better mate than the other males. To be chosen as a mate, 
males may perform courtship behaviors. These are special behaviors that help attract a mate. Male 
courtship behaviors get the attention of females and show off a male's traits. Different species have different 
courtship behaviors. Remember the peacock raising his tail feathers in Figure 1b? This is an example of 
courtship behavior. The peacock is trying to impress females of his species with his beautiful feathers. 

Another example of courtship behavior in birds is shown in Figure 7. This bird is called a blue-footed booby. 
He is doing a dance to attract a female for mating. During the dance, he spreads out his wings and stamps 
his feet on the ground. You can watch a video of a blue-footed booby doing his courtship dance at 
http://www.travelpod.com/travel-photo/harryandnorah/the_other_way/1199840760/blue-footed-booby- 
courting-dance.avi/tpod.html. 



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Figure 7: This blue-footed booby is a species of sea bird. The male pictured here is doing a courtship 
"dance." He is trying to attract a female for mating. 

(Source: http://en.wikipedia.Org/wiki/lmage:Blue-footed_Booby_%28Sula_nebouxii%29_-displaying.jpg, Li- 
cense: Creative Commons Attribution Share-Alike 2.0) 

Courtship behaviors occur in many other species. For example, males in some species of whales have 
special mating songs to attract females as mates. Frogs croak for the same reason. Male deer clash antlers 
to court females. Male jumping spiders jump from side to side to attract mates. To see a video of a jumping 
spider courting a mate, go to http://video.aol.com/video-detail/courtship-and-mating-of-the-jumping-spider- 
lyssomanes-viridis-araneae-salticidae/2837652909. 

Courtship behaviors are one type of display behavior. A display behavior is a fixed set of actions that carries 
a specific message. Although many display behaviors are used to attract mates, some display behaviors 
have other purposes. For example, display behaviors may be used to warn other animals to stay away, as 
you will read below. 

Caring for the Young 

In most species of birds and mammals, one or both parents care for their offspring. Caring for the young 
may include making a nest or other shelter. It may also include feeding the young and protecting them from 
predators. Caring for offspring increases their chances of surviving. When parents help their young survive, 
they increase their own fitness. 

Birds called killdeers have an interesting way to protect their chicks. When a predator gets too close to her 
nest, a mother killdeer pretends to have a broken wing. The mother walks away from the nest holding her 
wing as though it is injured. This is what the killdeer in Figure 8 is doing. The predator thinks she is injured 
and will be easy prey. The mother leads the predator away from the nest and then flies away. 



374 




Figure 8: This mother killdeer is pretending she has a broken wing. She is trying to attract a predator's at- 
tention in order to protect her chicks. This behavior puts her at risk of harm. How can it increase her fitness? 

(Source: http://farm1 .static.flickr.com/223/490628352_5c686bfe60.jpg?v=0, License: Unknown; information 
not provided on Web site.) 

In most species of mammals, parents also teach their offspring important skills. For example, meerkat parents 
teach their pups how to eat scorpions without being stung. A scorpion sting can be deadly, so this is a very 
important skill. Teaching the young important skills makes it more likely that they will survive. 

Defending Territory 

Some species of animals are territorial. This means that they defend their area. The area they defend usually 
contains their nest and enough food for themselves and their offspring. A species is more likely to be territorial 
if there is not very much food in their area. 

Animals generally do not defend their territory by fighting. Instead, they are more likely to use display behavior. 
The behavior tells other animals to stay away. It gets the message across without the need for fighting. 
Display behavior is generally safer and uses less energy than fighting. 

Male gorillas use display behavior to defend their territory. They pound on their chests and thump the ground 
with their hands to warn other male gorillas to keep away from their area. The robin in Figure 9 is also using 
display behavior to defend his territory. He is displaying his red breast to warn other robins to stay away. 



375 




Figure 9: The red breast of this male robin is easy to see. The robin displays his bright red chest to defend 
his territory. It warns other robins to keep out of his area. 

(Source: http://commons.wikimedia.org/wiki/lmage: American_Robin_2006.jpg, License: Creative Commons 
Attribution Share Alike 2.5) 

Some animals deposit chemicals to mark the boundary of their territory. This is why dogs urinate on fire 
hydrants and other objects. Cats may also mark their territory by depositing chemicals. They have scent 
glands in their face. They deposit chemicals by rubbing their face against objects. 

Cycles of Behavior 

Many animal behaviors change in a regular way. They go through cycles. Some cycles of behavior repeat 
each year. Other cycles of behavior repeat every day. 

Yearly Cycles 

An example of a behavior with a yearly cycle is hibernation. Hibernation is a state in which an animal's 
body processes are slower than usual and its body temperature falls. An animal uses less energy than usual 
during hibernation. This helps the animal survive during a time of year when food is scarce. Hibernation 
may last for weeks or months. Animals that hibernate include species of bats, squirrels, and snakes. 

Most people think that bears hibernate. In fact, bears do not go into true hibernation. In the winter, they go 
into a deep sleep. However, their body processes do not slow down very much. Their body temperature 
also remains about the same as usual. Bears can be awakened easily from their winter sleep. 

Another example of a behavior with a yearly cycle is migration. Migration is the movement of animals from 
one place to another. Migration is an innate behavior that is triggered by changes in the environment. For 
example, animals may migrate when the days get shorter in the fall. Migration is most common in birds, fish, 
and insects. In the Northern Hemisphere, many species of birds, including robins and geese, travel south 
for the winter. They migrate to areas where it is warmer and where there is more food. They return north in 
the spring. A flock of migrating geese is shown in Figure 10. 



376 




Figure 10: These geese are flying south for the winter. Flocks of geese migrate in V-shaped formations. 

(Source: http://commons.wikimedia.Org/wiki/lmage:BrantaLeucopsisMigration.jpg, License: Creative Commons 
Attribution Share Alike 2.5) 

Some animals migrate very long distances. The map in Figure 11 shows the migration route of a species 
of hawk called Swainson's hawk. About how many kilometers do the hawks travel from start to finish? Are 
you surprised that birds migrate that far? Some species of birds migrate even farther. 



Swainson's Hawk Migration Route 




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Figure 11: The migration route of Swainson's hawk starts in North America and ends in South America. 
Scientists learned their migration route by attaching tiny tracking devices to the birds. The birds were then 
tracked by satellite. On the migration south, the hawks travel about 8,000 kilometers from start to finish. 



377 



(Source: http://commons.wikimedia.org/wiki/lmage: Swainson%27s_hawk_migration_route.jpg, License: 
Public Domain) 

Birds and other migrating animals follow the same routes each year. How do they know where to go? It 
depends on the species. Some animals follow landmarks, such as rivers or coastlines. Other animals are 
guided by the position of the sun, the usual direction of the wind, or other clues in the environment. 

Daily Cycles 

Many animal behaviors change at certain times of day, day after day. For example, most animals go to sleep 
when the sun sets and wake up when the sun rises. Animals that are active during the daytime are called 
diurnal. Some animals do the opposite. They sleep all day and are active during the night. These animals 
are called nocturnal. Animals may eat and drink at certain times of day, as well. Humans have daily cycles 
of behavior, too. Most people start to get sleepy after dark and have a hard time sleeping when it is light 
outside. Daily cycles of behavior are called circadian rhythms. 

In many species, including humans, circadian rhythms are controlled by a tiny structure called the biological 
clock. This structure is located in a gland at the base of the brain. The biological clock sends signals to the 
body. The signals cause regular changes in behavior and body processes. The amount of light entering the 
eyes controls the biological clock. That's why the clock causes changes that repeat every 24 hours. 

Lesson Summary 

Communication is any way that animals share information. 

Social animals live together in groups and cooperate with one another. 

Some of the most important animal behaviors involve attracting mates and caring for offspring. 

Some animals defend the area where they live from other animals. 

Many animal behaviors occur in cycles that repeat yearly or daily. 

Review Questions 

1 . List two ways that animals communicate. (Beginning) 

2. Describe how ants in a colony cooperate. (Intermediate) 

3. What is courtship behavior? (Beginning) 

4. Why do male dogs urinate on fire hydrants and other objects? (Intermediate) 

5. Give an example of a circadian rhythm. (Intermediate) 

6. How do ants use chemicals to communicate? (Intermediate) 

7. Explain how courtship behaviors could evolve. (Challenging) 

8. How do adult animals increase their own fitness by teaching skills to their young? (Challenging) 

9. What is the advantage of animals using display behavior instead of fighting to defend their territory? (In- 
termediate) 

10. What is migration, and why do animals migrate? (Intermediate) 
Further Reading I Supplemental Links 

Bernard Stonehouse and Esther Bertram. The Truth about Animal Communications. Tangerine Press, 2003. 



378 



Betty Tatham. How Animals Communicate. Franklin Watts. 2004. 

Etta Kaner. Animal Groups (Animal Behavior). Tandem Library, 2004. 

Pamela Hickman. Animals and Their Mates: How Animals Attract, Fight for, and Protect Each Other. Kids 
Can Press, Ltd., 2004. 

Susan Glass. Staying Alive: Regulation and Behavior. Perfection Learning, 2005. 

http://encarta.msn.com/encyclopedia_761556353_3/Ant.html 

http://news.nationalgeographic.com/news/2003/07/0709_030709_socialanimals.html 

http://www.arts.uwa.edu.au/ data/page/108779/peacock_info.pdf 

http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm 

http://www.saczoo.com/3_kids/13_tales/_morecommunication_facial.htm 

http://www.usatoday.com/news/science/aaas/2002-04-05-coop-behavior.htm 

http://www.wjh.harvard.edu/~mnkylab/media/vervetcalls.html 

http://en.wikibooks.org 

Vocabulary 

biological clock Tiny structure in the brain that controls circadian rhythms. 

circadian rhythms An organism's daily cycles of behavior. 

communication Any way that animals share information. 

cooperation Working together with others. 

courtship behaviors Special behaviors that help attract a mate. 

display behavior Fixed set of actions that carries a specific message. 

hibernation State in which an animal's body processes are slower than usual. 

language Use of symbols (or sounds) to communicate. 

mating Pairing of an adult male and female to produce young. 

migration Movement of animals from one place to another; often seasonal. 

social animals Animals that live in groups with other members of their species. 

Review Answers 

1 . Animals communicate by means of (any two): sounds, gestures, facial expressions, and scents. 

2. Ants in a colony cooperate by taking different jobs and working together for the good of all. Worker ants 
build and repair the nest, find food, and care for the young. Soldier ants defend the colony against predators. 
The queen lays eggs, and drones mate with the queen. 

3. Courtship behavior is special behavior that helps attract a mate. For example, male courtship behaviors 
get the attention of females and show off the male's traits. 

4. Male dogs urinate on fire hydrants and other objects to mark the boundary of their territory with a chemical 
in their urine. Other dogs can smell the chemical. The scent of the chemical tells other dogs to stay away. 

5. An example of a circadian rhythm is the sleep-wake cycle that occurs in most animals. Like other circadian 
rhythms, it repeats every 24 hours. 



379 



6. Ants secrete many different chemicals. Other ants detect the chemicals with their antennae. Some of the 
chemicals signal all the ants in a colony to come together. Other chemicals warn of danger. Still others mark 
trails to food sources. 

7. Courtship behaviors help animals attract mates. Animals that are more successful at attracting mates are 
more likely to have offspring. Any trait that helps animals attract mates and have offspring will increase their 
fitness. The traits of these animals will increase in the species through natural selection. This is how courtship 
behaviors could evolve. 

8. When adult animals teach skills to their young, the young are more likely to survive. By helping their young 
survive, the adult animals' genes are more likely to pass on to the next generation. Therefore, the adult an- 
imals are increasing their own fitness by teaching skills to their young. 

9. Display behavior is a fixed set of actions that carries a specific message. Display behavior that is used 
to defend territory tells other animals to stay way. It gets the message across without the need for fighting. 
It is generally safer and uses less energy than fighting. 

1 0. Migration is the movement of animals from one place to another. It is an innate behavior that is triggered 
by changes in the environment, such as the days getting longer in the fall. Animals migrate to areas where 
it is warmer and there is more food. 

Points to Consider 

• The biological clock located just below the human brain controls behaviors such as the sleep-wake cycle. 

• The brain is part of the nervous system. What other body system are found in humans? 

• Which body system includes the bones? Which system includes the muscles? What do bones and 
muscles do? 



380 



16. Skin, Bones, and Muscles 



Organization of Your Body 

Lesson Objectives 

• List the levels of organization in the human body. 

• Identify the four types of tissues that make up the body. 

• Identify 12 organ systems. 

• Describe how organs and organ systems work together to maintain homeostasis. 

Check Your Understanding 

• What is a cell? 

• What are some of the differences between a prokaryotic cell and an eukaryotic cell? 

• What are some of the basic functions of animal cells? 

Introduction 

The men in Figure 1 have just jumped into freezing icy water. They are having fun, but imagine how cold 
they must feel! One minute their bodies were wrapped in warm clothes, the next, they were dunked in 
freezing water. Their bodies are now working hard to adapt to the sudden great change in temperature. The 
ability of the body to maintain a stable internal environment in the response to change is called homeostasis. 
Homeostasis allows your body to adapt to change, such as jumping into cold water, running in hot weather, 
or not getting enough food when you are hungry. Homeostasis is an important characteristic of living things. 




Figure 1: The bodies of these swimmers are working hard to maintain homeostasis while they are in the 
icy pool water. Otherwise, their life processes would stop working as soon as they got into the water.> 



381 



(Source: http://upload.wikimedia.Org/wikipedia/commons/8/89/lce_swimming_46.jpg, Photographer: Ch. 
Baltes, License: GFDL 1 .2) 

Cells, Tissues, and Organs 

Cells are the most basic units of life in your body. They must do many jobs to maintain homeostasis, but 
each cell does not have to do every job. Cells have specific jobs to maintain homeostasis. For example, 
nerve cells move electrical messages around the body, and white blood cells patrol the body and attack in- 
vading bacteria. There are many additional different types of cells. Other cells include red blood cells, skin 
cells, cells that line the inside of your stomach, and muscle cells. 

Groups of Cells Form Tissues 

Cells are grouped together to carry out specific functions. A group of cells that work together is called a tissue. 
Your body has four main types of tissues, as do the bodies of other animals. These tissues make up all 
structures and contents of your body. Figure 2 shows an example of each tissue type. 




Nervous (brain) 
(skeletal muscle) 



Epithelial (skin) 



x3k« ■>•.%-■■■ 




Connective (bone) Muscle 



Figure 2: Your body has four main types of tissue; nervous tissue, epithelial tissue, connective tissue, and 
muscle tissue. They are found throughout your body. 

(Sources: http://www.flickr.com/photos/lecates/313205903/, Photographer: dro!d, License: CC-BY-SA 2.0; 
http://training.seer.cancer.gov/module_anatomy/unit2_2_body_tissues1_epithelial.html, License: Public 
Domain; http://www.flickr.com/photos/goldenswamp/21 52200871/, Photographer: judy_breck, License: CC- 
BY-SA 2.0; http://upload.wikimedia.Org/wikipedia/commons/0/0c/Compact_bone_-_ground_cross_section.jpg, 
Photographer: Reytan, License: GFDL 1.2; http://upload.wikimedia.org/wikipedia/commons/5/56/Skele- 
tal_muscle_-_longitudinal_section.jpg, Photographer: Reytan, License: GFDL) 

• Epithelial tissue is made up of layers of tightly packed cells that line the surfaces of the body. Examples 
of epithelial tissue include the skin, the lining of the mouth and nose, and the lining of the digestive system. 

• Connective tissue is made up of many different types of cells that are all involved in structure and 
support of the body. Examples include blood, cartilage, and bone. 

• Muscle tissue is made up of cells that have filaments that move past each other and change the size 
of the cell. There are three types of muscle tissue: smooth muscle, skeletal muscle, and cardiac muscle. 



382 



• Nervous tissue is made up of the nerve cells that together form the nervous system. Nervous tissue is 
found in nerves, the spinal cord, and the brain. 

Groups of Tissues Form Organs 

A single tissue alone cannot do all the jobs that are needed to keep you alive and healthy. Two or more 
tissues working together can do a lot more. An organ is a structure made of two or more tissues that work 
together. The heart, shown in Figure 3, is made up of four types of tissues. 




Figure 3: The four different tissue types work together in the heart as they do in the other organs. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/d/d5/Heart_ myocardium_diagram.jpg, Medical Il- 
lustrators: Patrick J. Lynch and C. Carl Jaffe, M.D., License: CC-BY-SA 2.5) 

Groups of Organs Form Organ 

Systems Your heart pumps blood around your body. However, your heart needs to be able to get blood to 
and from every cell in your body in order to do its job. So, your heart is connected to blood vessels such as 
veins and arteries. Organs that work together form an organ system. Together, your heart, blood, and blood 
vessels form your cardiovascular system. 

Organ Systems Work Together 

Your body's 12 organ systems are shown in Figure 4. Your organ systems do not work alone in your body. 
They must all be able to work together to maintain homeostasis. For example, when the men in Figure 1 
jumped into the cold water, their integumentary systems (skin, hair, nails), cardiovascular systems, muscular 
systems, and nervous systems work quickly together to ensure the icy-cold water did not cause harm to 
their bodies. The nervous system sent nerve messages from the skin to tell the cardiovascular system to 
reduce the blood flow to the skin. Blood flow is then increased to the internal organs and large muscles to 
help keep them warm and supply them with oxygen. The nervous system also sent messages to the respi- 
ratory system to breathe faster. This allows for more oxygen to be delivered by the blood to the muscular 
system which is shivering and moving about to keep the body warm. Feedback loops in the nervous and 
endocrine systems regulate conditions in the body. A feedback loop is a path that leads from the initial 
generation of the signal to the subsequent modification of the initial event. For example, the men that jumped 
into the cold water do not need to continue to breathe faster and faster. Feedback loops return the respiratory 
system to "normal." 

Major Organ Systems of the Human Body 



383 



Organ System 



Major Tissues and 
Organs 



Function 



Example 



Cardiovascular 



Heart; blood vessels; 
blood 



transports oxygen, hormones and 
nutrients to the body cells, and 
wastes and carbon dioxide away from 
cells 




cardiovascular system, 



Lymphatic 



Lymph nodes; lymph 
vessels 



defense against infection and dis- 
ease, transfer of lymph between tis- 
sues and the blood stream 




Lymphatic Vasesis 



lymphatic system 



Digestive 



esophagus; stom- 
ach; small intestine; 
large intestine 



processing of foods and absorption 
of nutrients, minerals, vitamins, and 
water 



Large intestine 
(colon} 




digestive system 



384 



Endocrine 



pituitary gland, hy- 
pothalamus; adrenal 
glands; Islet of 
Langerhans; ovaries; 
testes 



communication within the body with 
hormones; directing long-term 
change over other organ systems to 
maintain homeostasis 




A 



endocrine system 



Integumentary 



Skin, hair, nails 



protection from injury and fluid loss; 
physical defense against infection by 
microorganisms; temperature control 



integumentary system 



Muscular 



cardiac (heart) mus- 
cle; skeletal muscle; 
smooth muscle; ten- 
dons 



movement, support, heat production 







muscular system 



Nervous 



brain, spinal cord; 
nerves 



collecting, transferring and process- 
ing information; directing short-term 
change over other organ systems in 
order to maintain homeostasis 




nervous system 



Reproductive 



Female: uterus; 
vagina; fallopian 
tubes; ovaries Male: 



production of gametes (sex cells) and 
sex hormones; production of offspring 



reproductive system 



385 



penis; testes; semi- 
nal vesicles 



Respiratory 



Trachea, larynx, 
pharynx, lungs 



delivery of air to sites where gas ex- 
change can occur between the blood 
and cells (around body) or blood and 
air (lungs) 




respiratory system 



Skeletal 



Bones, cartilage; lig- 
aments 



support and protection of soft tissues 
of body; movement at joints; produc- 
tion of blood cells; mineral storage 



*wr ! J 



n? 



.,.,.. 



skeletal system 



Urinary 



kidneys; 
bladder 



urinary 



removal of excess water, salts, and 
waste products from blood and body; 
control of pH; regulates water and 
electrolyte balence 




urinary system 



Immune 



Skin; bone marrow; 
spleen; white blood 
cells 



defending against microbial 
pathogens (disease-causing agents) 
and other diseases 



immune system 



Figure 4a: The Circulatory System. 



386 



(Source: http://upload.wikimedia.0rg/wikipedia/corr1mons/e/ec/Blutkreislauf.png, Photographer: User:San- 
sculotte, License: CC-by-SA 2.5) 

Figure 4b: The Lymphatic System. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/0/03/lllu_lymphatic_system.jpg, License: Public 
Domain) 

Figure 4c: The Digestive Tract. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/7/7d/Digestivetract.gif, License: Public Domain) 

Figure 4d: The Endocrine System. 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/d/da/lllu_endocrine_system.png, License: Public 
Domain) 

Figure 4e: 

Figure 4f: The Anterior Muscles. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/e/e5/Muscles_anterior_labeled.png, Image: User: 
Mikael Haggstrom, License: Public Domain) 

Figure 4g: The Nervous System. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/0/01/Nervous_system_diagram_%28dumb%29.png, 
Image: Persian Poet Gal License: Public Domain) 

Figure 4h: 

Figure 4i: The Respiratory System. 

(Source: http://commons.wikimedia.org/wiki/lmage: Respiratory_system_no.png, Image: Theresa Knott, Li- 
cense: CC-BY-SA 2.5, GFDL 1.2) 

Figure 4j: The Skeletal System. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Human_skeleton_frant_is.svg, Image: LadyofHats, License: 
Public Domain) 

Figure 4k: The Urinary System. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/5/53/lllu_urinary_system_neutral.png, License: 
Public Domain) 

Figure 41: 

Figure 4: Each body system works together to maintain homeostasis of other systems and of the entire 
organism. No system of the body works alone, and your well-being depends upon the well-being of all the 
body systems. A problem in one system usually affects other body systems. 

Lesson Summary 

• The levels of organization in the human body include: cells, tissues, organs, and organ systems. A tissue 
is a group of cells that work together. An organ is made of two or more tissues that work together. Organs 
that work together make up organ systems. 



387 



• There are four tissue types in the body: epithelial tissue, connective tissue, muscle tissue, and nervous 
tissue. There are 12 major organ systems in the body. Organs and organ systems work together to 
maintain homeostasis. 

Review Questions 

1 . What is homeostasis? (Beginning) 

2. What are the four levels of organization in an organism? (Beginning) 

3. What is the difference between a tissue and an organ? (Intermediate) 

4. List the four types of tissues that make up the human body. (Intermediate) 

5. A classmate says that all four tissue types are never found together in an organ. (Challenging) 

6. Why do you think an organ is able to do many more jobs than a single tissue can? (Challenging) 

7. Identify the organ system to which the following organs belong: skin, stomach, brain, lungs, and heart. 
(Intermediate) 

8. Give an example of how two organ systems work together to maintain homeostasis. (Intermediate) 
Further Reading I Supplemental Links 

http://en.wikipedia.org/wiki/Tissue_%28biology%29 
Vocabulary 

cardiovascular system The body system that include the heart, blood, and blood vessels. 

connective tissue Tissue that is made up of different types of cells that are involved in structure and 

support of the body; includes blood, bone, and cartilage. 

epithelial tissue A tissue that is composed of layers of tightly packed cells that line the surfaces 

of the body; examples of epithelial tissue include the skin, the lining of the mouth 
and nose, and the lining of the digestive system. 

homeostasis The ability of the body to maintain a stable internal environment in the response 

to external changes. 

muscular tissue Tissue that is composed of cells that have filaments that move past each other 

and change the size of the cell. There are three types of muscle tissue: smooth 
muscle, skeletal muscle, and cardiac muscle. 

nervous tissue Composed of nerve cells and related cells. 

organ A structure made of two or more tissues that work together. 

organ system A group of organs that work together. 

tissue A group of cells that work together for a common purpose. 

Review Answers 

1 . Homeostasis is the ability of the body to maintain a stable internal environment in the response to changes. 

2. Cell, tissue, organ, organ system 

3. A tissue is a group of cells that work together. An organ is two or more tissues that work together. 

4. Nervous tissue, connective tissue, muscle tissue, and epithelial tissue. 



388 



5. Do you agree with your classmate? Explain your answer. Sample answer: No, I do not agree. All four 
tissue types work together in body organs. 

6. An organ is made up of two or more types of tissues, so an organ can do many more jobs than a single 
tissue could do. 

7. Skin: Integumentary System 
Stomach: Digestive System 
Brain: Nervous system 
Lungs: Respiratory system 
Heart: Cardiovascular system 

8. Sample answer: When you are cold, your integumentary system sends messages through the nervous 
system to the cardiovascular system to reduce the blood flow to the skin. Instead, the warm blood gets sent 
to the internal organs to make sure they stay warm. 

Points to Consider 

• What are the levels of organization of the integumentary system? 

• What other body systems does the integumentary system work with to maintain homeostasis? 

The Integumentary System 

Lesson Objectives 

• List the functions of skin. 

• Describe the structure of skin. 

• Describe the structure of hair and nails. 

• Identify two types of skin problems. 

• Describe two ways to take care of your skin. 

Check Your Understanding 

• What is homeostasis? 

• What is epithelial tissue? 

Introduction 

Did you know that you see the largest organ in your body every day? You wash it, dry it, cover it up to stay 
warm or uncover it to cool off. In fact, you see it so often it is easy to forget the important role your skin plays 
in keeping you healthy. Your skin is part of your integumentary system, which is the outer covering of your 
body. The integumentary system is made up of your skin, hair, and nails. 



389 




Figure 1 : Skin acts as a barrier that stops water and other things, like soap and dirt, from getting into your 
body. 

(Source: http://www.flickr.com/photos/dankamminga/1238276965/, Image by: Dan Kamminga, License: CC- 
By-SA2.0) 

Your Skin and Homeostasis 

Your integumentary system has many roles in homeostasis, including protection, the sense of touch, and 
regulating body temperature. Keeping water out of the body is an important role for your integumentary 
system. If this were not so, the man in Figure 1 would not be able to bathe. All of your body systems work 
together to maintain stable internal conditions. Each of the parts that make up your integumantary system 
has a special role in maintaining homeostasis which we will explore a little later. 

Functions of Skin 

Your skin covers the entire outside of your body. Your skin is your body's largest organ yet it is only about 
2 mm thick. It has many important functions, some of these include: 

• It acts as a barrier. It keeps organisms that could harm the body out. It stops water from leaving the body, 
and stops water from getting into the body. 

• It helps regulate body temperature. It does this by making sweat, a watery substance which cools the 
body when it evaporates. 

• It helps you to gather information about your environment. Special nerve endings in your skin sense 
heat, pressure, cold and pain. 

• It helps the body get rid of some types of waste, which are removed in sweat. 

• It acts as a sun block. A chemical called melanin is made by certain skin cells when they are exposed 
to sunlight. Melanin blocks sun light from getting to deeper layers of skin cells, which are easily damaged 
by sun light. 

Structure of Skin 

Your skin is always exposed to your external environment so it gets cut, scratched, and worn down. You 
also naturally shed many skin cells every day. Your body replaces damaged or missing skin cells by growing 



390 



more of them. The layer of skin that you can see is actually dead. The dead cells are filled with a tough, 
waterproof protein called keratin. As the dead cells are shed or are removed from the upper layer, they are 
replaced by the skin cells below them. 

As you can see in Figure 2, two different layers make up the skin. These layers are the epidermis and the 
dermis. A fatty layer, called subcutaneous tissue, lies under the dermis, but it is not part of your skin. The 
layers that make up your skin are shown in Figure 2. 



hair shaft 



sweat pore 

dermal papilla 

sensory nerve ending 



for touch 



stratum corneuni 
pigment layer 

stratum germinativum 

{stratum spirosum 
stratum basalt 

orrL-ttor pili muscle 

sebaceous gland 

fiair follicle 



papilla of hair 



nc^vc liber 

blood -and 
lymph vessel* 




E-PIDE-RMIS 



l«H*1[S 



_SUBCUfI5 
(hypodermis) 



St»«at gland 

pacinian corpuscle 



Figure 2: Skin is made up of two layers, the epidermis on top, and the dermis below. The tissue below the 
dermis is called the hypodermis, but it is not part of the skin. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/3/34/Skin.jpg, License: Public Domain) 

The color, thickness and texture of skin vary over the body. There are two general types of skin; thin and 
hairy, which is the most common type on the body, and thick and hairless, which is found on parts of the 
body that experience a lot of friction, such as the palms of the hands or the soles of the feet. 

Epidermis 

Epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface 
and is made up of many layers of epithelial cells (discussed in lesson 1). The epidermis is divided into several 
layers where epithelial cells are formed by mitosis in the lowest layer. The epithelial cells move up through 
the layers of the epidermis, changing shape and composition as they divide and become filled with keratin. 
The skin cells at the surface of the epidermis form a thin layer of flattened, dead cells. Although the top layer 
of epidermis is only about as thick as a sheet of paper, it is made up of 25 to 30 layers of cells. 

The epidermis also contains cells that produce melanin. Melanin is the brownish pigment that gives skin 
and hair their color. Melanin-producing cells are found in the bottom layer of the epidermis. The epidermis 
does not have any blood vessels. The lower part of the epidermis is fed by diffusion from the blood vessels 



391 



of the dermis. 
Dermis 

The dermis is the layer of skin directly under the epidermis. It is made of a tough connective tissue that 
contains the protein collagen. Collagen is a long, fiber-like protein that is very strong. The dermis is tightly 
connected to the epidermis by a membrane made of collagen fibers. As you can see in Figure 2, the dermis 
contains the hair follicles, sweat glands, oil glands, and blood vessels. It also holds many nerve endings 
that give you your sense of touch, pressure, heat, and pain. Tiny muscles in the dermis pull on hair follicles 
which cause hair to stand up. This can happen when you are cold or afraid. The resulting little "bumps" in 
the skin are commonly called goosebumps, shown in Figure 3. 




Figure 3: Goose bumps are caused by tiny muscles in the dermis that pull on hair follicles, which causes 
the hairs to stand up straight. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/f/f1/2003-09-1 7_Goose_bumps.jpg, Image by: lldar 
Sagdejev, License: CC-By-SA 3.0, GFDL 1 .2) 

Oil Glands and Sweat Glands 

Glands and follicles open out into the epidermis, but they start in the dermis. Oil glands secrete an oily 
substance, called sebum, into the hair follicle. An oil gland is shown in Figure 2. Sebum "waterproofs" hair 
and the skin surface to prevent them from drying out. It can also stop the growth of bacteria on the skin. 
Sebum is the cause of the oily appearance of skin and hair. It is odorless, but the breakdown of sebum by 
bacteria can cause odors. If an oil gland becomes plugged and infected, it develops into a pimple, also 
called acne. 

Sweat glands open to the skin surface through skin pores. They are found all over the body. Evaporation 
of sweat from the skin surface helps to lower the skin temperature, which in turn helps to control body tem- 
perature. The skin also releases excess water, salts, and other wastes in sweat. A sweat gland is shown in 
Figure 2. 

Nails and Hair 

Nails and hair are made of the same types of cells that make up skin. Hair and nails contain the tough protein 
keratin. Both hair and nails are important parts of your integumentary system. 



392 



Fingernails and toenails both grow from nail beds. A nailbed is thickened to form a lunula (or little moon), 
which you can see in Figure 4. Cells forming the nail bed are linked together to form the nail. As the nail 
grows more cells are added at the nail bed. Older cells get pushed away from the nail bed and the nail grows 
longer. There are no nerve endings in the nail, which is a good thing, otherwise cutting you nails would hurt 
a lot! 

Nails act as protective plates over the fingertips and toes. Fingernails also help in sensing the environment. 
The area under your nail has many nerve endings, which allow you to receive more information about objects 
you touch. Nails are made up of many different parts, as shown in Figure 4. 




Figure 4: The structure of fingernails is similar to toenails. The free edge is the part of the nail that extends 
past the finger, beyond the nail plate. The nail plate is what we think of when we say "nail," the hard portion 
made of the tough protein keratin. The lunula is the crescent shaped whitish area of the nail bed. The cuticle 
is the fold of skin at the end of the nail. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Fingernail_label.jpg, License: Public Domain) 

Hair sticks out from the epidermis, although it grows from hair follicles deep in the dermis, as shown in Figure 
5. Hair is made of keratin, the same protein that makes up skin and nails. Hair grows from inside the hair 
follicle. New cells grow in the bottom part of the hair, called the bulb. Older cells get pushed up, and the hair 
grows longer. Similar to nails and skin, the cells that make up the hair strand are dead and filled with keratin. 
Hair color is the result of different types of melanin in the hair cells. In general, the more melanin in the cells, 
the darker the hair color; the less melanin, the lighter the hair color. 




Hair 



Skin surface 



Sebum 
Follicle 

Sebaceo 
gland^ 



393 



Figure 5: Hair, hair follicle, and oil glands. The oil, called sebum, helps to prevent water loss from the skin. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/4/49/ HairFollicle.png, License: Public Domain) 

Hair helps to keep the body warm. When you feel cold, your skin gets a little bumpy. These bumps are 
caused by tiny muscles that pull on the hair, causing the hair to stick out. The erect hairs help to trap a thin 
layer of air that is warmed by body heat. In mammals that have much more hair than humans, the hair traps 
a layer of warm air near the skin and acts like warm blanket. Hair also protects the skin from ultraviolet radi- 
ation (UV radiation) from the sun. Hair also acts as a filter. Nose hair helps to trap particles in the air that 
may otherwise travel to the lungs. Eyelashes shield eyes from dust and sunlight. Eyebrows stop salty sweat 
and rain from flowing into the eye. 

Keeping Skin Healthy 

Some sunlight is good for health. Vitamin D is made in the skin when it is exposed to sunlight. But, getting 
too much sun can be unhealthy. A sunburn is a burn to the skin that is caused by overexposure to UV radi- 
ation from the sun's rays or tanning beds. Light-skinned people, like the girl in Figure 6, get sunburned more 
quickly than people with darker skin do. This is because melanin in the skin acts as a natural sunblock that 
helps to protect the body from UV radiation. When exposed to UV radiation, certain skin cells make melanin, 
which causes skin to tan. Children and teens who have gotten sunburned are at a greater risk of developing 
skin cancer later in life than children and teens who have not. 

Long-term exposure to UV radiation is the leading cause of skin cancer. About 90 percent of skin cancers 
are linked to sun exposure. UV radiation damages the genetic material of skin cells. This damage can cause 
the skin cells to grow out of control and form a tumor. Some of these tumors are very difficult to cure. For 
this reason you should always wear sunscreen with a high sun protection factor (SPF), a hat, and clothing 
when out in the sun. As people age, their skin gets wrinkled. Wrinkles are caused mainly by UV radiation 
and by the loosening of the connective tissue in the dermis due to age. 




Figure 6: Sunburn is caused by overexposure to UV rays. Getting sunburned as a child or a teen, especially 
sunburn that causes blistering, increases the risk of developing skin cancer later in life. 

(Source: http://commons.wikimedia.org/wi ki/lmage:Sunbum_(1 3141 7495). jpg, Image by: Kelly Sue DeCon- 
nick, License: CC-By-SA 2.0) 

Because some types of skin cancer are easy to cure, the dangers of too much sunlight are not always taken 
seriously by people. It is important to remember that a more serious form of skin cancer, called melanoma, 
is also associated with long-term sun exposure. Melanomas are difficult to treat, and potentially deadly tumors. 
The best way to avoid skin cancer is to cover up when outside in the sun, and to wear sunscreen. 

Bathing and Skin Hygiene 

During the day, your skin can collect many different things. Sweat, oil, dirt, dust, and dead skin cells can 
build upon the skin surface. If not washed away, the mix of sweat, oil, dirt, and dead skin cells can encourage 
the excess growth of bacteria. These bacteria feed on these substances and cause a smell that is commonly 



394 



called body odor. Dirty skin is also more prone to infection. Bathing every day helps to remove dirt, sweat 
and extra skin cells, and helps to keep your skin clean and healthy. 

Acne 

Hormones can affect your skin. Certain hormones cause oil glands in the skin to make an oil called sebum. 
When too much sebum is made by oil glands, it can cause the hair follicles to get blocked with dead skin 
cells. Within these blocked pores bacteria and yeast begin to multiply. In response to the growth of the 
bacteria and yeast, the skin inflames. This skin inflammation produces the red bumps that are called acne. 
Upto85%ofteenagersgetacne. Acne usually goes away by adulthood. Frequent washing can help reduce 
the amount of sebum and dead skin cells on the skin. But washing cannot prevent the excessive sebum 
production that leads to acne. 

Injury 

Your skin can heal itself even after a large cut. Cells that are damaged or cut away are replaced by cells 
that grow in the bottom layer of the epidermis and the dermis. These new cells will eventually replace the 
damaged tissues. 

When an injury is deep enough through the epidermis into the dermis, bleeding occurs. A blood clot and 
scab soon forms. After the scab is formed, cells in the base of the epidermis begin to divide by and move 
to the edges of the scab. A few days after the injury, the edges of the wound are pulled together. If the cut 
is large enough, the production of new skin cells will not be able to heal the wound. Stitching the edges of 
the injured skin together can help the skin to repair itself. The person in Figure 7 had a large cut that needed 
to be stitched together. When the damaged cells and tissues are replaced, the stitches will be removed. 




Figure 7: Sewing the edges of a large cut together allows the body to repair the damaged cells and tissues, 
and heal the tear in the skin. 



395 



(Source: http://www.flickr.com/photos/wfryer/904561925/in/set- 72157601018537832/, Image by: Wesley 
Fryer, License: CC-SA 2.0) 

Lesson Summary 

Skin acts as a barrier that keeps particles and water out of the body. 

The skin helps to cool the body in hot temperatures, and keep the body warm in cool temperatures. It 
also help you to sense your surroundings. 

Skin is made up of two layers, the epidermis and the dermis. Hair and nails are made of the same type 
of protein as skin is. 

Nails grow from nail beds and hairs grow from hair follicles in the skin. 

Acne is a skin problem that happens when the skin makes too much sebum. 

Skin cancer can be caused by excess exposure to ultraviolet light from the sun or tanning beds. 

Bathing frequently helps keep the skin clean and healthy. 

Wearing sunblock and a hat when outdoors can help prevent skin cancer. 

Review Questions 

1 . Identify two functions of skin. (Beginning) 

2. How does the integumentary system help maintain homeostasis? (Beginning) 

3. Describe the structure of skin. (Beginning) 

4. Identify the layer of skin from which hair grows. (Intermediate) 

5. In what way are hairs and nails similar to skin? (Beginning) 

6. Name two functions of nails. (Beginning) 

7. Name two functions of hair. (Beginning) 

8. What type of skin problem happens when the skin makes too much sebum?(Beginning) 

9. The World Health Organization recommends that no person younger than 18 years old use a tanning 
bed. Why do you think using a tanning bed is not recommended? (Challenging) 

10. How does washing your skin help to keep you healthy? (Beginning) 

11 . Why are stitches sometimes needed if a person gets a deep or long cut in their skin? (Intermediate) 
Further Reading I Supplemental Links 

http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5540a9.htm 
http://www.cdc.gov/Features/SkinCancer 



396 



http://en.wikipedia.org/wiki 
Vocabulary 

body odor Smell that is produced by the breakdown of sweat by bacteria that live on the 

skin. 

dermis The layer of skin directly under the epidermis; made of a tough connective tissue 

that contains the protein collagen. 

epidermis The outermost layer of the skin; forms the waterproof, protective wrap over the 

body's surface; made up of many layers of epithelial cells. 

integumentary system The outer covering of the body; made up of the skin, hair, and nails. 

keratin Tough, waterproof protein that is found in epidermal skin cells, nail, and hair. 

melanin The brownish pigment that gives skin and hair their color. 

melanocyte Melanin-producing cells; found in the bottom layer of the epidermis. 

melanoma Cancer of melanin-containing cells (melanocytes); mostly linked to long-term ex- 

posure to UV radiation. 

oil gland Skin organ that secretes an oily substance, called sebum, into the hair follicle. 

subcutaneous tissue Fatty layer of tissue that lies under the dermis, but is not part of the skin. 

sunburn A burn to the skin that is caused by overexposure to UV radiation from the sun's 

rays or tanning beds. 

sweat gland Gland that opens to the skin surface through skin pores; found all over the body; 

secretes sweat. 

Review Answers 

1. Sample answer: Skin prevents foreign particles from getting into the body. It also helps keep the body 
cool in hot weather. 

2. The skin helps to cool the body when it is too hot, and hair helps keep the body warm when it is cold. 

3. Skin has two layers, the epidermis and the dermis. 

4. Hairs grow from the dermis. 

5. Hair and nails are made of keratin, the same type of protein that skin is made of. 

6. Sample answer: Nails help protect the fingers and toes from bumps and scrapes, and also help in sensing 
the environment. 

7. Sample answer: Hair helps to keep the body warm, and it also protects the head from the sun's UV radi- 
ation. 

8. Acne results when the skin makes too much sebum. 

9. Tanning beds are a source of UV radiation. The UV radiation of a tanning bed causes the same type of 
damage to skin cells as UV radiation from the sun does. Getting a sunburn as a child or teenager has been 
linked to an increased risk of skin cancer in later life. 

10. Washing your skin removes dirt, bacteria, old skin cells, and other things that might cause the skin to 
get sore or infected. 

11. The skin would not be able to heal such a large wound without the edges of the wound being much 
closer together. Stitching the edges of a large wound together gives the body a better chance to repair the 



397 



damaged cells and tissues. 

Points to Consider 

• How might what you eat affect your bones? 

• What do you think is the most important function of your skeletal system? 

Skeletal System 

Lesson Objectives 

• Identify the main tissues and organs of the skeletal system. 

• List four functions of the skeletal system. 

• Describe three movable joints. 

• Identify two nutrients that are important for a healthy skeletal system. 

• Describe two skeletal system injuries. 

Check Your Understanding 

• What is an organ system? 

• What is connective tissue? 

Introduction 

How important is your skeleton? Can you imaging your body without it? You would be a wobbly pile of 
muscle and internal organs, and you would not be able to move around much. You will learn about these 
functions in this lesson. Your skeleton is important for many different things. Bones are the main organs of 
the skeletal system. They are made up of living tissue. If you think you have broken a bone it's important to 
visit a healthcare professional. A broken bone may not heal properly by itself. A sprain can be bandaged 
up properly to reduce swelling and discomfort. A doctor or other healthcare professional can also give you 
advice on how to manage such an injury at home. 

Your Skeleton 

Humans are vertebrates, which are animals that have a backbone. The sturdy scaffolding of bones and 
cartilage that is found inside vertebrates is called a skeleton. The adult human skeleton has about 206 
bones, some of which are named in Figure 1 . The skeletons of babies and children have many more bones 
and more cartilage than adults have. As a child grows, these "extra" bones grow into each other, and cartilage 
gradually hardens to become bone tissue. 

You may think that bones are dry and lifeless, but they are very much alive. The white, hard bones that you 
might see in a museum or science book, are only the hard mineral remains of the bone tissue. Living bones 
are full of life. They contain many different types of tissues. 

Cartilage is found at the end of bones and is made of tough protein fibers called collagen. Cartilage creates 
smooth surfaces for the movement of bones that are next to each other, like the bones of the knee. Ligaments 
are made of tough protein fibers and connect bones to each other. Your bones, cartilage, and ligaments 
make up your skeletal system. 



398 



Spiral Colurrn 
Cervical 
Vertebral .**■ 

Tbomefc 



Lurrthar 



S-.iI 




Circle 
MBfwbfkim 
Scapula 
Sternum 

Pits 



Hume 




M 






Ulna 
Radws 



&an___; 

Metacarpals 
:i "i;: .!!:;■:■■ 



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Metalarsals 



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Figure 1 : The skeletal system is made up of bones, cartilage, and ligaments. The skeletal system has many 
important functions in your body. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Human_skeleton_front.svg, Image: LadyofHats Mariana 
Ruiz Villarreal, License: Public Domain-self) 

Functions of Bones 

Your skeletal system gives shape and form to your body, but it is also important in other homeostatic functions. 
The main functions of the skeletal system are: 

• Support The skeleton supports the body against the pull of gravity. The large bones of the lower limbs 
support the trunk when standing. 



399 



• Protection The skeleton provides a framework that supports and protects the soft organs of the body. 
For example, the skull surrounds the brain to protect it from injury. The bones of the rib cage help protect 
the heart and lungs. 

• Movement Bones work together with muscles as simple mechanical lever systems to move the body. 

• Making Blood Cells Blood cells are made mostly inside certain types of bones. 

• Storage Bones store calcium. They contain more calcium than any other organ does. Calcium is released 
by the bones when blood levels of calcium drop too low. The mineral phosphorus is also stored in bones. 

Structure of Bones 

Bones are organs. Recall that organs are made up of two or more types of tissues. Bones come in many 
different shapes and sizes, but they are all made of the same materials. The two main types of bone tissue 
are compact bone and spongy bone. Compact bone makes up the dense outer layer of bones. Spongy 
bone is lighter and less dense than compact bone, and is found toward the center of the bone. The tough, 
shiny, white membrane that covers all surfaces of bones is called the periosteum. 

Many bones also contain a soft connective tissue called bone marrow. There are two types of bone marrow: 
red marrow and yellow marrow. Red marrow makes red blood cells, platelets, and most of the white blood 
cells for the body (discussed in the Diseases and the Body's Defenses chapter). Yellow marrow makes 
white blood cells. The bones of newborn babies contain only red marrow. As children get older, their red 
marrow is replaced by yellow marrow. In adults, red marrow is found mostly in the bones of the skull, the 
ribs, and pelvic bones. Bone come in four main shapes. They can be long, short, flat, or irregular. Identifying 
a bone as long, short, flat, or irregular is based on the shape of the bone not the size of the bone. For example, 
both small and large bones can be classified as long bones. The small bones in your fingers and the largest 
bone in your body, the femur, are long bones. Figure 2 shows the structure of a long bone. 



Long Bone 

Epiphysis 



Diaphysis 




Articular cartilage 
Epiphyseal line 

Spongy bone 

Medullary cavity 
Nutrient foramen 



Endosteurn 
Periosteum 



I" / 

Epiphysis yy 



Articular cartilage 



Figure 2: Bones are made up of different types of tissues. 



400 



(Source: http://upload.wikimedia.Org/wikipedia/commons/9/94/lllu_long_bone.jpg, License: Public Domain) 

How Bones Develop and Grow 

Your skeleton began growing very early in your development. After only eight weeks of growth from a fertilized 
egg, your skeleton was formed by cartilage and other connective tissues. At this point your skeleton was 
quite bendy and flexible. After a few more weeks of growth, the cells that form hard bone began growing in 
the cartilage, and your skeleton began to harden. However, not all of the cartilage is replaced by bone. 
Cartilage remains in many places in your body including your joints, your rib cage, your ears, and the tip of 
your nose. 

A baby is born with zones of cartilage in its bones that allow growth of the bones. These areas, called growth 
plates, allow the bones to grow longer as the child grows. When the child reaches an age of about 18 to 25 
years, all of the cartilage in the growth plate is replaced by bone. This stops the bone from growing any 
longer. 

Even though bones stop growing in length in early adulthood, they can continue to increase in thickness 
throughout life. This thickening can be in response to stress from increased muscle activity or to weight- 
bearing exercise. 

Joints and How They Move 

A joint is a point at which two or more bones meet. There are three types of joints in the body: fixed, partly 
movable, and movable. Fixed joints do not allow any bone movement. Many of the joints in your skull are 
fixed. Partly movable joints allow only a little movement. Your backbone has partly movable joints between 
the vertebrae. Movable joints allow movement and provide mechanical support for the body. Joints are a 
type of lever, which is a rigid object that is used to increase the mechanical force that can be applied to another 
object. Can openers and scissors are examples of levers. Joints reduce the amount of energy that is spent 
moving the body around. Just imagine how difficult it would be to walk about if you did not have knees! 



Fused joints 




Figure 3a: The skull has fused joints. Fused joints do not allow any movement of the bones, which protects 
the brain from injury. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/f/fa/SkullSideView.jpg, Image: Bernard bill5, License: 
GFDL1.2) 



401 



Vertebral Column 



Cervical vertebrae 
Thoracic vertebrae 




Sacrum 



Coccygeal vertebrae 



Cervical curve 



Thoracic curve 



Lumbar curve 



Sacral curve 



Figure 3b: The joints between your vertebrae (b) are partially movable. 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/f/f8/lllu_vertebral_column.jpg, License: Public 
Domain) 

Movable Joints 

Movable joints are the most mobile joints of all. They are also the most common type of joint in your body. 
Your fingers, toes, hips, elbows, and knees all have movable joints. The surfaces of bones at movable joints 
are covered with a smooth layer of cartilage. The space between the bones in a movable joint is filled with 
a liquid called synovial fluid. Synovial fluid is a thick, stringy fluid that looks a lot like egg white. The fluid lu- 
bricates and cushions the bones when they move at the joint. There are many different types of movable 
joints, and many different examples. Four types of movable joints are shown in Figures 4- 6. 

In a ball and socket joint the ball-shaped surface of one bone fits into the cuplike shape of another. Examples 
of a ball and socket joint include the hip, shown in Figure 4, and the shoulder. 



UPPER END 
OF THIGH BONE 




HIP 




Figure 4: Your hip joint is a ball and socket joint. The "ball" end of one bone fits into the "socket" of another 
bone. These joints can move in many different directions. 

(Sources: http://upload.wikimedia.Org/wikipedia/commons/c/c4/Socket_1_%28PSF%29.png. Image: Pearson 
Scott Foresman, License: Public Domain; http://upload.wikimedia.org/wikipedia/commons/4/4b/Gelenke_Ze- 
ichnung01.jpg, Image: Produnis, License: GFDL) 

In a hinge joint, the ends of the bones are shaped in away that allows motion only in two directions, forward 
and backward. Examples of hinge joints are the knees and elbows. A knee joint is shown in Figure 5. 



402 





Front view 



Side view 



Figure 5: The knee joint is a hinge joint. Like a door hinge, a hinge joint allows backward and forward 
movement. 

(Sources: http://commons.wikimedia.0rg/wiki/lmage:Knee_skeleton_lateral_anterior_views.svg, Image: 
Patrick J. Lynch and C. Carl Jaffe, M.D., License: CC-BY-SA 2.5; http://upload.wikimedia.org/wikipedia/com- 
mons/4/4b/Gelenke_Zeichnung01.jpg, Image: Produnis, License: GFDL) 

The pivot joint is formed by a process that rotates within a ring, the ring being formed partly of bone, and 
partly of ligament. An example of a pivot joint is the joint between the radius and ulna that allows you to turn 
the palm of your hand up and down. A pivot joint is shown in Figure 6. 

A gliding joint is a joint which allows only gliding movement. The gliding joint allows one bone to slide over 
the other. The gliding joint in your wrist allows you to flex your wrist. It also allows you to make very small 
side-to-side motions. There are also gliding joints in your ankles. 





Figure 6: Pivot Joint The joint at which the radius and ulna meet is a pivot joint. Movement at this joint allows 
you to flip your palm over without moving your elbow joint. 

(Sources: http://en.wikipedia.Org/wiki/lmage:MedialHumerusRadiusUlnaArticulated.jpg.jpg, Photographer: 
Brian C. Goss, License: Public Domain; http://upload.wikimedia.org/wikipedia/commons/4/4b/Gelenke_Ze- 
ichnung01.jpg, Photographer: Produnis, License: GFDL) 

Keeping Bones and Joints Healthy 

Just like a houseplant depends on you taking good care of it by watering it and giving it plant food, so too 
does your body depend on you! You can help keep your bones and skeletal system healthy by eating well 
and getting enough exercise. Weight-bearing exercises help keep bones strong. Weight-bearing exercises 
work against gravity; such activities include basketball, tennis, gymnastics, karate, running, and walking. 
When the body is exercised regularly by doing a weight-bearing activity, bones respond by adding more 



403 



bone cells making the bones denser. 

Eating Well 

Did you know that what you eat now, as a teenager, can affect how healthy your skeletal system will be in 
30, 40, and even 50 years from now? Calcium and vitamin D are two of the most important nutrients for a 
healthy skeletal system. Your bones need calcium to grow properly. If you do not get enough calcium in 
your diet as a teenager, your bones may become weak and break easily later in life. Osteoporosis is a 
disease in which bones become lighter and more porous than they should be. Light and porous bones are 
more likely to break, which can cause pain and prevent a person from walking. Being immobile can cause 
more bone loss which can make the disease worse. 

Older women are most likely to develop osteoporosis because it is linked to the decrease in production of 
sex hormones. However, poor nutrition, especially diets that are low in calcium and vitamin D, increase the 
risk of osteoporosis in later life. Not doing regular weight-bearing exercises is also linked to having thinner, 
weaker bones. Two of the easiest ways to prevent osteoporosis is to eat a healthful diet that has the right 
amount of calcium and vitamin D, and to do weight-bearing exercise day. 

Foods that are a good source of calcium include: milk, yogurt, and cheese. Non-dairy sources of calcium 
include Chinese cabbage, kale, and broccoli. Many fruit juices, fruit drinks, tofu, and cereals have calcium 
added into them. These foods are also an important source of calcium. 

Teenagers are recommended to get 1 300 mg of calcium every day. One cup of milk provides about 300 mg 
of calcium, which about 30% of your daily requirement for calcium. Figure 7 shows other sources of calcium. 





1 1 / s ounces of Cheddar cheese 



6 fluid ounces of orange 
juice 



The amont of calcium in 
1 cup (8 fl.oz.)of milk 

is 

about equal to the amont 





2 Vi cups of cooked broccoli 



3 ounces of tinned sardines(with 
bones) 



Figure 7: There are many different sources of calcium. Getting enough calcium in your daily diet is important 
for good bone health. How many ounces of Cheddar cheese would provide your recommended daily intake 
of calcium? 

(Sources: http://commons.wikimedia.Org/wiki/lmage:Milk_glass.jpg, Image: Stefan Kiihn, License: GFDL; 
http://commons.wikimedia.Org/wiki/lmage:Bravo_Cheddar.jpg, License: Public Domain; http://upload.wikime- 
dia.org/wikipedia/commons/6/6d/Broccoli_in_a_dish_1 .jpg, Image: Quadell, License: GFDL 1 .2, CC-BY-SA 
2.0; http://upload.wikimedia.Org/wikipedia/commons/f/fd/OrangeJuice_1.jpg, License: Public Domain) 

Your skin makes vitamin D when exposed to sunlight. The pigment melanin in the skin acts like a filter that 
can prevent the skin from making vitamin D. As a result, people with darker skin need more time in the sun 
than people with lighter skin to make the same amount of vitamin D. 

Fish is naturally rich in vitamin D. Vitamin D is added to other foods including milk, soy milk and breakfast 
cereals. Teenagers are recommended to get 5 micrograms (200 lU/day) of vitamin D everyday. A 3 bounce 
portion of cooked salmon provides 360 IU of vitamin D. 



404 



Lack of vitamin D, or deficiency, can be caused by two different things: not enough sunlight exposure, and 
lack of vitamin D in the diet. Vitamin D deficiency results in problems with bone growth and hardening. This 
leads to bone softening diseases such as rickets in children and osteomalacia in adults. Osteomalacia is a 
bone disease in which the bones do not harden properly and they can break easily. Rickets is a type of os- 
teomalacia. Figure 8 is an X-ray of a child that has rickets. Lack of vitamin D may also be related to osteo- 
porosis. 




Figure 8: Rickets is a softening of the bones in children that may cause fractures and bending of the bones, 
especially of the legs. The bones have not hardened properly because of lack of vitamin D, and bend under 
the weight of the body. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/a/a9/XrayRicketsLegssmall.jpg, Image: Mrich, Li- 
cense: CC-BY-SA) 

Bone Fractures 

Even though they are very strong, bones can fracture, or break. Fractures can happen at different places 
on a bone. They are usually due to excess bending stress on the bone. Bending stress is what causes a 
pencil to break if you were to bend it too far. Soon after a fracture, the body begins to repair the break. The 
area becomes swollen and sore. Within a few days bone cells travel to the break site and begin to rebuild 
the bone. It takes about 2 to 3 months before compact and spongy bone form at the break site. 

Sometimes the body needs extra help in repairing a broken bone. In such a case a surgeon will piece a 
broken bone together with metal pins. Moving the broken pieces together will help keep the bone from 
moving, and give the body a chance to repair the break. A broken ulna has been repaired with pins in Figure 
9. 



405 




Figure 9: The upper part of the ulna, at the elbow is broken, as you can see in the X-ray at left. The x-ray 
at right was taken after a surgeon inserted pins into the joint to keep the two pieces of the ulna together. 
The two pieces of bone were reattached with metal pins. The line of staples is closing the skin wound. 

(Source: http://upl0ad.wikimedia.0rg/wikipedia/c0mm0ns/c/cl /Fracture_of_Olecranon_pre_and_post_typi- 
cal_surgery.jpg, Image: Michael Muller-Hillebrand, License: CC-BY-SA 3.0) 

Cartilage Injuries 

Osteoarthritis is a condition in which the cartilage at the ends of the bones breaks down. The break down 
of the cartilage leads to pain and stiffness in the joint. Decreased movement of the joint because of the pain 
may lead to muscles that are attached to the joint to become weaker, and ligaments may become looser. 
Osteoarthritis is the most common form of arthritis. It has many causes, some of the more common causes 
include old age, sport injuries to the joint, bone fractures, and overweight and obesity. Total hip replacement 
is a common treatment for osteoarthritis. 

Ligament Injuries 

Recall that a ligament is a short band of tough connective tissue that connects bones together to form a 
joint. Ligaments can get injured when a joint gets twisted or bends too far. The protein fibers that make up 
a ligament can get strained or torn, causing swelling and pain. Injuries to ligaments are called sprains. Ankle 
sprains are a common type of sprain. A small ligament in the knee, called the anterior cruciform ligament 
(ACL), is a common site of injury in athletes. Ligament injuries can take a long time to heal. Treatment of 
the injury includes rest and special exercises that are developed by a physical therapist. 

Preventing Injuries 

Preventing injuries to your bones and ligaments is easier and much less painful than treating an injury. 
Wearing the correct safety equipment when doing activities that require safety equipment can help prevent 
many common injuries. For example, wearing a bicycle helmet can help prevent a skull injury if you fall. 
Warming up and cooling down properly can help prevent ligament and muscle injuries. Torn ligaments and 
fractured bones are common sport injuries. Such injuries need to be treated by a doctor. Overuse injuries 
such as ligament strains and tears are common injuries for teenage athletes. Correct conditioning and 
enough rest can help prevent overuse injuries. 

Stretching after activity may help prevent injuries. Regular stretching improves the flexibility of muscles and 
tendons. It also improves the range of mobility of your joints. Stretching can also improve your posture, and 
may help prevent some aches and pains caused by tight muscles. 

Lesson Summary 

• Bones, cartilage, and ligaments make up the skeletal system. The skeleton supports the body against 
the pull of gravity. The skeleton provides a framework that supports and protects the soft organs of the 



406 



body. Bones work together with muscles as simple mechanical lever systems to move the body. Blood 
cells are made mostly inside the bone marrow. Bones store calcium. 

• There are three types of joints in the body: fixed, partly movable, and movable. Fixed joints do not allow 
any bone movement. Partly movable joints allow only a little movement. Movable joints allow movement 
and provide mechanical support for the body. Joints are a type of lever, which is a rigid object that is 
used to increase the mechanical force that can be applied to another object. Joints reduce the amount 
of energy that is spent moving the body around. Calcium and vitamin D are two of the most important 
nutrients for a healthy skeletal system. 

• Bones need calcium to grow properly. Vitamin D deficiency results in problems with bone growth and 
hardening. Osteoporosis is a disease in which bones become lighter and more porous than they should 
be. Light and porous bones are more likely to break than dense bones. Osteomalacia is a bone disease 
in which the bones do not harden properly and they can break easily. Osteoarthritis is a condition in 
which the cartilage at the ends of the bones breaks down. The break down of the cartilage leads to pain 
and stiffness in the joint. A sprain is an injury to a ligament. A fracture is a break or crack in a bone. 

Review Questions 

1 . What are the organs of the skeletal system? (Beginning) 

2. Name one tissue of the skeletal system. (Beginning) 

3. List four functions of the skeletal system. (Beginning) 

4. Name three types of movable joints. (Beginning) 

5. "All joints in the body are movable." Do you agree with this statement? Explain why or why not. (Interme- 
diate) 

6. How are the joints in your body similar to levers? (Intermediate) 

7. Why is calcium important for a healthy skeletal system? (Intermediate) 

8. The recommended daily amount of calcium for teenagers is 1300 mg. If a person gets only 1000 mg a 
day, what percentage of the recommended daily amount are they getting? (Intermediate) 

9. Name two things you can do to keep your skeletal system healthy. (Beginning) 

10. What part of the skeletal system does osteoarthritis affect? (Intermediate) 

11 . Why might a doctor need to insert pins into a broken bone? (Intermediate) 
Further Reading I Supplemental Links 

http://www.girlshealth.gov/bones 

http://www.cdc.gov/nccdphp/dnpa/nutrition/nutrition_for_everyone/basics/calcium.htm 

http://en.wikipedia.org/wiki 

Vocabulary 

ball and socket joint Joint structure in which the ball-shaped surface of one bone fits into the cuplike 
depression in another bone; examples include the shoulder and hip joints. 

bone marrow Soft connective tissue found inside many bones; site of blood cell formation. 



407 



cartilage 

compact bone 
fracture 

gliding joint 

hinge joint 

joint 

ligaments 

movable joint 

osteoarthritis 

osteoporosis 

periosteum 

pivot joint 

skeletal system 
skeleton 
spongy bone 
sprain 



Smooth covering found at the end of bones; made of tough collagen protein fibers; 
creates smooth surfaces for the easy movement of bones against each other. 

The dense, hard outer layer of a bone. 

Bone injury, often called a "break;" usually caused by excess bending stress on 
bone. 

Joint structure that allows one bone to slide over the other; examples includes the 
joints in the wrists and ankles. 

Joint structure in which the ends of bones are shaped in a way that allows motion 
in two directions only (forward and backward); examples include the knees and el- 
bows. 

Point at which two or more bones meet. 

Fibrous tissue that connects bones to other bones; made of tough collagen fibers. 

Most mobile type of joint; the most common type of joint in the body. 

A condition in which the cartilage at the ends of the bones breaks down. 

Disease in which bones become lighter and more porous than normal. 

Tough, shiny, white membrane that covers all surfaces of bones. 

Joint structure in which the end on one bone rotates within a ring-type structure 
which can be made partly of bone and partly of ligament; example includes the joint 
between the radius and ulna. 

Body system that is made up of bones, cartilage, and ligaments. 

Sturdy scaffolding of bones and cartilage that is found inside vertebrates. 

Lighter and less dense than compact bone; found toward the center of the bone. 

A ligament injury; usually caused by the sudden overstretching of a joint which 
causes tearing. 



Review Answers 

1 . Bones are the organs of the skeletal system. 

2. Sample answer: Compact bone. Other tissues include spongy bone, periosteum, marrow, and cartilage. 

3. To protect the body organs; give support to the body; allow movement; and to make new blood cells. 

4. A ball-and-socket joint, a hinge joint, and a sliding joint. 

5. Sample answer: No, I do not agree. Some joints in the body do not move, such as the joints between the 
bones of the skull. 

6. Joints reduce the amount of energy that is spent moving the body around in the same way that levers 
reduce the amount of energy needed to do a certain job. 

7. Your bones need calcium to grow properly. If a person does not get enough calcium in their diet, bones 
may become weak and break easily later in life. 

8. 1000/1300 x 100 = 76.9 % (or 77%) 

9. Eat the right amount of calcium and vitamin D, and do weight-bearing exercises most days of the week. 

10. Osteoarthritis is a breakdown in the cartilage. 



408 



11. If the pieces of bone are far away from each other, or are broken into many pieces, pinning the bone 
together will allow the body to repair the damage more easily. 

Points to Consider 

• How does your skeletal system interact with your muscular system? 

• How might a broken bone affect the functioning of the muscular system? 

• How do tendons differ from ligaments? How are they similar? 

The Muscular System 

Lesson Objectives 

• Identify the three muscle types in the body. 

• Describe how skeletal muscles and bones work together to move the body. 

• Describe how exercise affects the muscular system. 

• Identify two types of injuries to the muscular system. 

Check Your Understanding 

• What is muscle tissue? 

• What is the function of the muscular system? 

Introduction 

The muscular system is the body system that allows us to move. You depend on many muscles to keep 
you alive. Your heart, which is mostly muscle, pumps blood around your body. Muscles are always moving 
in your body. Certain muscle movements happen without you thinking about them, while you can control 
other muscle movements. In this lesson you will learn about the different types of muscles in your body and 
how your muscular system works with the other body systems to keep you alive and healthy. You will also 
learn how and why regular physical activity is important for good health. 

Types of Muscles 

Each muscle in the body is made up of cells called muscle fibers. Muscle fibers are long, thin cells that can 
do something that other cells cannot do — they are able to get shorter. Shortening of muscle fibers is called 
contraction. Nearly all movement in the body is the result of muscle contraction. 

You are aware of and can control certain muscle movements. Other muscle movements you are not aware 
of and cannot control. Muscles that you can control are called voluntary muscles. Muscles that you cannot 
control are called involuntary muscles. There are three different types of muscles in the body: skeletal, 
smooth, and cardiac muscle. Skeletal muscle is voluntary muscle. Smooth muscle and cardiac muscle are 
involuntary muscles. 

• Skeletal muscle is usually attached to the skeleton. Skeletal muscles move the body. They usually 
contract voluntarily, but they can contract involuntarily by reflexes. For example, you can choose to move 
your arms, but your arm would move automatically if you were to burn your finger on a stove top. 

• Smooth muscle is found within the walls of organs and structures such as the esophagus, stomach, 
intestines, and blood vessels. Unlike skeletal muscle, smooth muscle is involuntary muscle which means 
it not under your control. 



409 



Cardiac muscle is also an involuntary muscle but is a specialized kind of muscle found only in the heart. 




Cardiac muscle is found in the 
heart. It contracts involuntarily. 



Skeletal muscle works with bones 
to move the body. 



Smooth muscle moves food 
through your digestive system. 



Figure 1: There are three types of muscles in the body: cardiac, skeletal, and smooth. Everyone has the 
same three types of muscle tissue, no matter their age. 

(Sources: http://www.flickr.com/photos/bike/1380483811/, Image: richardmasoner, License: CC-BY-SA2.0; 
http://en.wikipedia.Org/wiki/lmage:Glanzstreifen.jpg, Image: Dr. S. Girod and Anton Becker, License: GFDL; 
http://commons.wikimedia.0rg/wiki/lmage:Skeletal_muscle_-_ longitudinal_section.jpg, Image: Reytan, 
Courtesy: Department of Histology, Jagiellonian University Medical College, License: GFDL; http://upload.wiki- 
media.org/wikipedia/commons/3/3b/Glatte_ Muskelzellen.jpg, Image: Polarlys, License: GFDL, CC-BY-SA 
2.0) 

Muscles, Bones, and Movement 

Skeletal muscles are attached to the skeleton by tendons. A tendon is a tough band of connective tissue 
that connects a muscle to a bone. Tendons are similar to ligaments except that ligaments join bone to each 
other. Muscles move the body by contracting against the skeleton. When muscles contract they get shorter, 
when they relax, they get longer. By contracting and relaxing, muscles pull on bones and allow the body to 
move. Muscles work together in pairs. Each muscle in the pair works against the other to move bones at 
the joints of the body. The muscle that contracts to cause a joint to bend is called the flexor. The muscle 
that contracts to cause the joint to straighten is called the extensor. 

For example, the biceps and triceps muscles work together to allow you to bend and straighten your elbow. 
Your biceps muscle, shown in Figure 2, contracts, and at the same time the triceps muscle relaxes. The 
contracting biceps pull on the radius bone and the elbow bends. To straighten the arm, the biceps muscle 
relaxes and the triceps on the opposite side of the elbow joint contracts. The biceps is the flexor and the 
triceps is the extensor of your elbow joint. In this way the joints of your body act like levers. This lever action 
of your joints reduces the amount of energy you have to spend to make large body movements. 



410 




TRICEPS 



Figure 2: The biceps and triceps act against one another to bend and straighten the elbow joint. To bend 
the elbow, the biceps contract and the triceps relax. To straighten the elbow, the triceps contract and the 
biceps relax. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/5/5f/Biceps_ %28PSF%29.jpg, License: Public 
Domain-self) 

Muscles and the Nervous System 

Muscles are controlled by the nervous system (see the Controlling the Body chapter). Nerves send messages 
to the muscular system from the brain. Nerves also send messages to the brain from the muscles. Remember 
that smooth and cardiac muscles are involuntary muscles. This means that you cannot control the nerve 
messages that get sent to and from these muscles. For example, you cannot make your heart muscle stop 
beating. Likewise, you cannot make food stop moving through your intestines. You can however control the 
movement of your skeletal muscle. When you want to move your foot, electrical messages called impulses 
move along nerve cells from your brain to the muscles of your foot. At the point at which the nerve cell and 
muscle cells meet, the electrical message is converted to a chemical message. The muscle cells receive 
the chemical message, which causes tiny protein fibers inside the muscle cells to get shorter. The muscles 
contract, pulling on the bones, and your foot moves. 

Muscles and Exercise 

Your muscles are important for carrying out everyday activities. The ability of your body to carry out your 
daily activities without getting out of breath, sore, or overly tired is called physical fitness. Physical fitness 
also describes the body's ability to respond to emergencies and to avoid getting sick. A person can have a 
good level of physical fitness or a poor level of fitness. For example, a person who becomes breathless and 
tired after climbing a flight of stairs is not physically fit. 

Physical exercise is any activity that maintains or improves physical fitness and overall health. Regular 
physical exercise is important in preventing lifestyle diseases such as heart disease, cardiovascular disease, 
Type 2 diabetes, and obesity. 

Regular exercise improves the health of the muscular system. Muscles that are exercised are bigger and 
stronger than muscles that are not exercised. Exercise improves both muscular strength and muscular en- 
durance. Muscular strength is the ability of a muscle to exert force during a contraction. Muscular en- 
durance is the ability of a muscle to continue to contract over a long time without getting tired. Two types 
of exercises help improve the fitness of muscles, anaerobic exercise and aerobic exercise. 



411 



Exercises are grouped into three types depending on the effect they have on the body: 

• Aerobic exercises such as cycling, walking, and running, increase muscular endurance. 

• Anaerobic exercises such as weight training, or sprinting increase muscle strength. 

• Flexibility exercises such as stretching, improve the range of motion of muscles and joints. Regular 
stretching helps avoid activity-related injuries. 

Anaerobic Exercise and Muscular Strength 

Anaerobic exercises cause muscles to get bigger and stronger. Anaerobic exercises use a resistance 
against which the muscle has to work to lift or push away. The resistance can be a weight or a person's own 
body weight, as shown in Figure 3. As a result of repeated muscle contractions, muscle fibers build up larger 
energy stores and the muscle tissue gets bigger. The larger a muscle is the greater the force it can apply 
to lift a weight or move a body joint. The muscles of weight lifters are large, and are therefore strong. 




Figure 3: Anaerobic exercises involve the muscles working against resistance. In this case the resistance 
is the person's own body weight. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/2/24/ Seatedl_egRaise.jpg, Image: GeorgeStepanek, 
License: GFDL 1.2) 

Aerobic Exercise and Muscular Endurance 

Aerobic exercises are exercises that cause your heart to beat faster and allow your muscles to use oxygen 
to contract. Aerobic exercise causes many different changes in skeletal muscle. Muscle energy stores are 
increased, the ability to use oxygen improves, and more capillaries surround the muscle fibers. These 
changes result in the ability of the muscle to avoid getting tired, and to use oxygen and food more efficiently. 
Aerobic exercise also helps improve cardiac muscle. It results in the heart being able to pump a larger volume 
of blood with each beat due to an increase in the size of the heart's ventricles. Examples of an aerobic ex- 
ercise are shown in Figure 4. 




Figure 4: When done regularly, aerobic activities such as cycling, make the heart stronger. 

(Source: http://www.flickr.com/photos/mindfrieze/764505669/, Image: mindfrieze, License: CC-BY-SA 2.0) 



412 



Both aerobic and anaerobic exercises also improve the ability of the heart to pump blood around the body. 
Aerobic exercise causes the heart to get bigger, and anaerobic exercise causes the walls of the heart to get 
thicker. These changes allow the heart to push more blood throughout the body with every heartbeat. 

Keeping Muscles Healthy 

Being physically active for 60 minutes a day for at least five days a week improves your physical fitness. 
Being physically active can also help you to reduce your risk of developing diseases such as cardiovascular 
disease, Type 2 diabetes, obesity, and certain forms of cancer. Being physically active does not mean you 
have to do boring workouts. You do not have to join a gym or be in a sports team to be physically active. 
Physical activities can include everyday things such as walking your dog, vacuuming or sweeping, cycling 
to school, skating, or climbing a flight of stairs. 




Figure 5: Adding more physical activity in your daily life does not mean boring or expensive activities, it can 
be fun! Local and community pools often run swim classes that do not cost a lot, and are designed for be- 
ginners. 

(Source: http://www.flickr.eom/photos/northfield_mn/1 88277425/in/ set-721 575941 97251 930/, Image: 
Northfield.org, License: CC-BY-NC-SA) 

Muscle Injuries 

Sometimes muscles and tendons get injured when a person starts doing an activity before they have warmed 
up properly. A warm up is a slow increase in the intensity of a physical activity that prepares muscles for 
an activity. Warming up increases the blood flow to the muscles and increases the heart rate. Warmed-up 
muscles and tendons are less likely to get injured. For example, before running or playing soccer, a person 
might jog slowly to warm muscles and increase their heart rate. It is important that warm ups prepare the 
muscles that are to be used in the activity. Even elite athletes need to warm up, as shown in Figure 6. Some 
injuries are caused by overuse. An overuse injury happens if the muscle or joint is not rested enough between 
activities. 

A strain is an injury to a muscle in which the muscle fibers tear because the muscle contracts too much or 
contracts before the muscle is warmed up. Strains are also known as pulled muscles. Overuse injuries often 
involve tendons. Overuse of tendons can cause tiny tears within the protein fibers of the tendon, which 
gradually weakens the tissue. These tiny tears lead to swelling, pain, and stiffness; a condition called ten- 
dinitis. Tendinitis can affect any tendon that is overused. Strains and tendinitis are usually treated with rest, 
cold compresses, and stretching exercises that a physical therapist designs for each patient. 



413 



' ' m .Si 




Figure 6: Warming up before the game helps the players avoid injuries. Some warm-ups may include 
stretching exercises. Some researchers believe stretching before activities may help prevent injury. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/3/30/WM06_ASA-UKR_Warm_Up.jpg, Image: 
Photocopy, License: CC-BY-SA 2.0) 

Proper rest and recovery are also as important to health as exercise is. If you do not get enough rest, your 
body will become injured and will not improve or react well to exercise. It is important to remember to allow 
enough recovery time for muscles and tendons to rest between exercise sessions. You can rest muscles 
by doing a different activity to what you normally do. For example, if you run, you can rest your running 
muscles and joints by swimming. This type of rest is called active rest. 

Anabolic steroids are hormones that cause the body to build up more protein in its cells. Muscle cells, 
which contain a lot of protein, get bigger when exposed to anabolic steroids. Your body naturally makes 
small amounts of anabolic steroids. They help your body repair from injury, and help to build bones and 
muscles. Anabolic steroids are used as medicines to treat people that have illnesses that affect muscle and 
bone growth. However, some people who do not need anabolic steroid as medicine try to increase their 
muscle size by taking these steroids. When taken in this way, anabolic steroids can have long-term affects 
other body systems. They can damage the person's kidneys, heart, liver, and reproductive system. If taken 
by adolescents, anabolic steroids can cause bones to stop growing, resulting in stunted growth. 

Lesson Summary 

• The body has three types of muscle tissue: skeletal, cardiac, and smooth. Muscles move the body by 
contracting against the skeleton. Muscles are controlled by the nervous system. 

• Nerves send messages to the muscular system from the brain. Nerves also send messages to the brain 
from the muscles. Regular exercise improves the health of the muscular system. Muscles that are exer- 
cised are bigger and stronger than muscles that are not exercised. 

• Exercise improves both muscular strength and muscular endurance. Muscular strength is the ability of 
a muscle to exert force during a contraction. Muscular endurance is the ability of a muscle to continue 
to contract over a long time without getting tired. Identify two types of injuries to the muscular system. 

• A strain is an injury to a muscle in which the muscle fibers tear because the muscle contracts too much 
or contracts before the muscle is warmed up. Tiny tears and swelling in a tendon results in tendinitis. 

Review Questions 

1 . Name the three types of muscle tissue in the body. (Beginning) 

2. Which of the three types of muscles in the body are voluntary? (Beginning) 



414 



3. What is another name for muscle cells? (Intermediate) 

4. Describe how skeletal muscles and bones work together to move the body. (Intermediate) 

5. What is a tendon? (Intermediate) 

6. How does aerobic exercise affect the heart? (Intermediate) 

7. How does aerobic exercise affect skeletal muscle? (Beginning) 

8. How does anaerobic exercise affect skeletal muscle? (Beginning) 

9. What is a muscle strain? (Intermediate) 

10. Why is warming up before exercise a good idea? (Intermediate) 

11 . Why are taking anabolic steroids a dangerous way to try to build up muscles? (Intermediate) 
Further Reading I Supplemental Links 
http://www.hmc.psu.edu/healthinfo/rn/musclestrain.htm 
http://www.cdc.gov/nccdphp/dnpa/physical/everyone/index.htm 

http://en.wiki.org 
Vocabulary 



aerobic exercises 

anabolic steroids 
anaerobic exercise 

cardiac muscle 
contraction 
extensor 

flexibility exercises 
flexor 
involuntary muscle 

muscle cells 
muscle fibers 
muscular endurance 
muscular strength 
muscular system 
physical exercise 
physical fitness 

skeletal muscle 
smooth muscle 

strain 



Types of exercises that cause the heart to beat faster and allow the muscles to 
obtain energy to contract by using oxygen. 

Hormones that cause the body to build up more protein in its cells. 

Types of exercises that involve short bursts of high-intensity activity; forces the 
muscles to obtain energy to contract without using oxygen. 

An involuntary and specialized kind of muscle found only in the heart. 

Shortening of muscle fibers. 

The muscle that contracts to cause a joint to straighten. 

Stretching exercises that improve the range of motion of muscles and joints. 

The muscle that contracts to cause a joint to bend. 

A muscle that a person cannot consciously control; cardiac muscle and smooth 
muscle are involuntary. 

Long, thin cells that can contract; also called muscle fibers. 

Long, thin cells that can contract; also called muscle cells. 

The ability of a muscle to continue to contract over a long time without getting tired. 

The ability of a muscle to exert force during a contraction. 

The body system that allows movement. 

Any activity that maintains or improves physical fitness and overall health. 

The ability of your body to carry out your daily activities without getting out of breath, 
sore, or overly tired. 

The muscle that is usually attached to the skeleton. 

Involuntary muscle found within the walls of organs and structures such as the 
esophagus, stomach, intestines, and blood vessels. 

An injury to a muscle in which the muscle fibers tear because the muscle contracts 
too much or contracts before the muscle is warmed up. 



415 



tendinitis A condition in which tiny tears form in the protein fibers of the tendon and gradually 

weaken the tissue. 

tendon A tough band of connective tissue that connects a muscle to a bone. 

voluntary muscle A muscle that a person can consciously control; skeletal muscle is voluntary. 

warm-up A slow increase in the intensity of a physical activity that prepares muscles for an 

activity. 

Review Answers 

1. Cardiac muscle, smooth muscle, and skeletal muscle 

2. Skeletal is the only voluntary muscle, and the other two types are involuntary. 

3. Another name for muscle cells is muscle fibers. 

4. Skeletal muscles contract and pull on the bones to make the bones move. 

5. A tendon is a tough band of connective tissue that attaches muscles to bones. 

6. Aerobic exercise causes the heart to get bigger, which allows the heart to push more blood throughout 
the body with every heartbeat. 

7. Aerobic exercise increases muscular endurance. 

8. Anaerobic exercises increase muscle strength. 

9. A strain is an injury to a muscle in which the muscle fibers tear because the muscle contracts too much 
or contracts before the muscle is warmed up. 

1 0. A warm up is a slow increase in the intensity of a physical activity that prepares muscles for an activity. 
Warmed-up muscles and tendons are less likely to get injured. 

11. Anabolic steroids have dangerous side effects. The steroids can cause damage to the heart, kidney, 
and reproductive system. They can also stop bones from growing. 

Points to Consider 

• How does your muscular system depend on your digestive system? 

• How does what you choose to eat affect your muscular system and your skeletal system? 



416 



17. Food and the Digestive System 



Food and Nutrients 

Lesson Objectives 

• Explain why the body needs food. 

• Identify the roles of carbohydrates, proteins, and lipids. 

• Give examples of vitamins and minerals, and state their functions. 

• Explain why water is a nutrient. 

Check Your Understanding 

What are the four types of organic compounds? 

What do all cells need in order to function? 

What are muscles made of? 

introduction 

Did you ever hear the old saying "An apple a day keeps the doctor away"? Do apples really prevent you 
from getting sick? Probably not, but eating apples and other fresh fruits can help keep you healthy. The girl 
shown in Figure 1 is eating fresh vegetables as part of a healthy meal. Why do you need foods like these 
for good health? What roles does food have in the body? 




Figure 1 : This girl is eating a salad of tomatoes and leafy green vegetables. Fresh vegetables such as these 
are excellent food choices for good health. 

(Source: http://www.ehponline.org/docs/2006/114-2/saladgirl.jpg, License: Public Domain) 

Why We Need Food 

Your body needs food for three reasons: 

• Food gives your body energy. You need energy for everything you do. 



417 



• Food provides building materials for your body. Your body needs building materials so it can grow and 
repair itself. 

• Food contains substances that help control body processes. Your body processes must be kept in balance 
for good health. For example, your body needs the right balance of water and salts. 

For all these reasons, you must have a steady supply of nutrients. Nutrients are chemicals in food that your 
body needs. There are six types of nutrients: carbohydrates, proteins, lipids, vitamins, minerals, and water. 
Carbohydrates, proteins, and lipids give your body energy. Proteins provide building materials. Proteins, 
vitamins, and minerals help control body processes. 

Nutrients that Provide Energy 

Molecules of carbohydrates, proteins, and lipids contain energy. When your body digests food, it breaks 
down the molecules of these nutrients. This releases the energy so your body can use it. The energy in food 
is measured in units called Calories. 

Carbohydrates 

Carbohydrates are nutrients that include sugars, starches, and fiber. Figure 2 shows how many grams of 
carbohydrates you need each day. It also shows some foods that are good sources of carbohydrates. 

High-Carbohydrate Foods 



Fresh fruits are good 
sources of simple 
carbohydrates. An apple 
has about 20 grams of 
carbohydrates. 




Whole grain breads are 
good sources of complex 
carbohydrates. A slice of 
whole grain bread has 
about 1 5 grams of 
carbohydrates. 



Vegetables are good 
sources of complex 
carbohydrates. A cup of 
cooked acorn squash 
has about 30 grams of 
carbohydrates. 



Figure 2: Up to the age of 13 years, you need about 130 grams of carbohydrates a day. Most of the carbo- 
hydrates should be complex. They are broken down by the body more slowly than simple carbohydrates. 
Therefore, they provide energy longer and more steadily. What other foods do you think are good sources 



418 



of complex carbohydrates? 

(Sources: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Vari0us_apples.jpg, License: Public Domain; 
http://c0mm0ns.wikimedia.0rg/wiki/lmage:Mixed_bread_l0aves.jpg, License: Creative Commons Attribution 
Sharealike 2.0; http://commons.wikimedia.0rg/wiki/lmage:Acornsquash.jpg, License: Public Domain; 
http://whatscookingamerica.net/NutritionalChart.htm, http://www.annecollins.com/dietary-carbs/carbs-in- 
veggies.htm http://www.carbohydrate-counter.org/, http://www.iom.edU/Object.File/Master/21/372/0.pdf) 

Sugars are small, simple carbohydrates that are found in foods such as fruits and milk. The sugar found in 
fruits is called fructose. The sugar found in milk is called lactose. These sugars are broken down by the 
body to form glucose, the simplest sugar of all. Glucose is used by cells for energy. Remember the discussion 
of cellular respiration in the Cell Functions chapter? Cellular respiration turns glucose into the usable form 
of chemical energy, ATP. One gram of sugar provides your body with four Calories of energy. 

Some people cannot digest lactose, the sugar in milk. This condition is 
called lactose intolerance. If people with this condition drink milk, they 
may have cramping, bloating, and gas. To avoid these symptoms, they should 
not drink milk, or else they should drink special, lactose-free milk. 



Starches are large, complex carbohydrates. They are found in foods such as vegetables and grains. Starches 
are broken down by the body into sugars that provide energy. Like sugar, one gram of starch provides your 
body with four Calories of energy. 

Fiber is another type of large, complex carbohydrate. Unlike sugars and starches, fiber does not provide 
energy. However, it has other important roles in the body. There are two types of fiber found in food: soluble 
fiber and insoluble fiber. Each type has a different role. 

• Soluble fiber dissolves in water. It helps keep sugar and fat at normal levels in the blood. 

• Insoluble fiber does not dissolve in water. As it moves through the large intestine, it absorbs water. This 
helps keep food waste moist so it can pass easily out of the body. 

Eating foods high in fiber helps fill you up without providing too many Calories. Most fruits and vegetables 
are high in fiber. Some examples are shown in Figure 3. 



419 



High-Fiber Foods 




Figure 3: Between the ages of 9 and 13 years, girls need about 26 grams of fiber a day, and boys need 
about 31 grams of fiber a day. Do you know other foods that are high in fiber? 

(Sources: http://www.dsf.health.state.pa.us/health/lib/health/broccoli.jpg, License: Public Domain; 
http://commons.wikimedia.Org/wiki/lmage:Green_peas.jpg, License: Creative Commons Attribution 2.5; 
http://commons.wikimedia.0rg/wiki/lmage:PearPhoto.jpg, License: GNU Free Documentation; http://com- 
mons.wikimedia.org/wiki/lmage:Avocado.jpg, License: Public Domain; http://www.bellaonline.com/arti- 
cles/art49482.asp, http://www.iom.edU/Object.File/Master/21/372/0.pdf, http://www.mayoclinic.com/health/high- 
fiber-foods/NU00582) 

Proteins 

Proteins are nutrients made up of smaller molecules called amino acids. As discussed in the Introduction 
to Living Things chapter, the amino acids are arranged like "beads on a string." These amino acid chains 
then fold up into a three-dimensional molecule. Proteins have several important roles in the body. For example, 
proteins: 

• Make up muscles. 

• Help control body processes. 

• Help the body fight off bacteria and other "foreign invaders." 

• Carry substances in the blood. 

If you eat more proteins than you need for these purposes, the extra proteins are used for energy. One gram 
of protein provides four Calories of energy. This is the same amount as one gram of sugar or starch. Figure 
4 shows how many grams of proteins you need each day. It also shows some foods that are good sources 
of proteins. 



420 



High-Protein Foods 



An 8-ounce glass of milk 
has about 8 grams of 
proteins. 



A 3-ounce serving of 
chicken has about 20 
grams of proteins. 




A cup of kidney beans 
has about 16 grams of 
proteins. 



Figure 4: Between the ages of 9 and 13 years, you need about 34 grams of proteins a day. What other 
foods do you think are good sources of proteins? 

(Sources: http://commons.wikimedia.Org/wiki/lmage:Milk_glass.jpg, License: GNU Free Documentation; 
http://www.kikkomanusa.com/_images/uploaded_images/420C6A.jpg, License: GNU Free Documentation; 
http://commons.wikimedia.Org/wiki/lmage:DarkRedKidney.jpg, License: Public Domain; 

http://www.iom.edU/Object.File/Master/21/372/0.pdf, http://www.youngwomenshealth.org/protein.html) 

There are many different amino acids, the building blocks of proteins, but your body needs only 20 of them. 
Your body can make ten of these amino acids from simpler substances. The other ten amino acids must 
come from the proteins in foods. These ten are called essential amino acids. Only animal foods, such as 
milk and meat, contain all ten essential amino acids in a single food. Plant foods are missing one or more 
essential amino acids. However, by eating a combination of plant foods, such as beans and rice, you can 
get all ten essential amino acids. 

Lipids 

Lipids are nutrients such as fats that store energy. The heart and skeletal muscles rely mainly on lipids for 
energy. One gram of lipids provides nine Calories of energy. This is more than twice the amount provided 
by carbohydrates or proteins. Lipids have several other roles in the body. For example, lipids: 

• Protect nerves. 

• Help control blood pressure. 

• Help blood to clot. 

• Make up the membranes that surround cells. 

Fats are one type of lipid. Fat is the main form in which the body stores energy. Stored fat gives your body 
an energy reserve. It's like having money in a savings account. It's there in case you need it. Stored fat also 
cushions and protects internal organs. In addition, it insulates the body. It helps keep you warm in cold 



421 



weather. 

Fats and other lipids are necessary for life. However, they can be harmful if you eat too much of them, or 
the wrong type of fats. Fats can build up in the blood and damage blood vessels. This increases the risk of 
heart disease. There are two types of lipids: saturated lipids and unsaturated lipids. 

• Saturated lipids are harmful even in very small amounts. They should be avoided as much as possible. 
Saturated fats are found mainly in animal foods, such as meats, whole milk, and eggs. Saturated fats 
increase cholesterol levels in the blood. Cholesterol is a fatty substance that is found naturally in the 
body. Too much cholesterol in the blood can lead to heart disease. It is best to limit the amount of saturated 
fats in your diet. 

• Unsaturated lipids are found mainly in plant foods, such as vegetable oil, olive oil, and nuts. Unsaturated 
lipids are also found in fish such as salmon. Unsaturated lipids are needed in small amounts for good 
health because your body cannot make them. Most lipids and fats in your diet should be unsaturated. 

Another type of lipid is called trans fat. Trans fat is manufactured and added 
to certain foods to keep them fresher for longer. Foods that contain trans 
fats include cakes, cookies, fried foods, and margarine. Eating foods that 
contain trans fats increases the risk of heart disease. You should do your 
best to eat fewer foods that contain it. Beginning in 2010, California will 
ban trans fats from restaurant products, and, beginning in 2011, from all 
retail baked goods . 



Vitamins and Minerals 

Vitamins and minerals are also nutrients. They do not provide energy. However, they are needed for good 
health. 

Vitamins 

Vitamins are substances that the body needs in small amounts to function properly. Humans need 13 dif- 
ferent vitamins. Some of them are listed in Table 1. The table also shows how much of each vitamin you 
need each day. Vitamins have many roles in the body. For example, Vitamin A helps maintain good vision. 
Vitamin B 9 helps form red blood cells. Vitamin K is needed for blood to clot when you have a cut or other 

wound. 

Table 1: Vitamins Needed For Good Health 



Vitamin 


One Reason You Need It 


Some Foods that Have It 


How Much of It You Need 
Each Day (at ages 9-13 
years) 


Vitamin A 


Needed for good vision 


Carrots, spinach, milk, 
eggs 


600ug(1 ug = 1x10" 6 g) 


Vitamin B 1 


Needed for healthy nerves 


Whole wheat, peas, meat, 
beans, fish, peanuts 


0.9 mg (1 mg = 1 x 10" 3 g) 


Vitamin B 3 


Needed for healthy skin 
and nerves 


Beets, liver, pork, turkey, 
fish, peanuts 


12 mg 


Vitamin B g 


Needed to make red blood 
cells 


Liver, peas, dried beans, 
green leafy vegetables 


300 ug 


Vitamin B 12 


Needed for healthy nerves 


Meat, liver, milk, shellfish, 
eggs 


1.8 ug 


Vitamin C 


Needed for growth and re- 
pair of tissues 


Oranges, grapefruits, red 
peppers, broccoli 


45 mg 



422 



Vitamin D 



Needed for healthy bones 
and teeth 



Milk, salmon, tuna, eggs 



5ug 



Vitamin K 



Needed for blood to clot 



Spinach, Brussels sprouts, 
milk, eggs 



60 ug 



Some vitamins are produced in the body. For example, vitamin D is made in the skin when it is exposed to 
sunlight. Vitamins B 12 and K are produced by bacteria that normally live inside the body. Most other vitamins 

must come from foods. Foods that are good sources of vitamins are listed in Table 1. They include whole 
grains, vegetables, fruits, and milk. 

Not getting enough vitamins can cause health problems. For example, too little vitamin C causes a disease 
called scurvy. People with scurvy have bleeding gums, nosebleeds, and other symptoms. Getting too much 
of some vitamins can also cause health problems. The vitamins to watch out for are vitamins A, D, E, and 
K. These vitamins are stored by the body, so they can build up to high levels. Very high levels of these vitamins 
can even cause death, although this is very rare. 

Minerals 

Minerals are chemical elements that are needed for body processes. Minerals that you need in relatively 
large amounts are listed in Table 2. Minerals that you need in smaller amounts include iodine, iron, and zinc. 
Minerals have many important roles in the body. For example, calcium and phosphorus are needed for 
strong bones and teeth. Potassium and sodium are needed for muscles and nerves to work normally. 

Table 2: Minerals Needed For Good Health 



Mineral 


One Reason You Need It 


Some Foods that Have It 


How Much of It You Need 
Each Day (at ages 9-13 
years) 


Calcium 


Needed for strong bones 
and teeth 


Milk, soymilk, green leafy 
vegetables 


1,300 mg 


Chloride 


Needed for proper balance 
of water and salts in body 


Table salt, most packaged 
foods 


2.3 g 


Magnesium 


Needed for strong bones 


Whole grains, green leafy 
vegetables, nuts 


240 mg 


Phosphorus 


Needed for strong bones 
and teeth 


Meat, poultry, whole 
grains\ 


1 ,250 mg 


Potassium 


Needed for muscles and 
nerves to work normally 


Meats, grains, bananas, 
orange juice 


4.5 g 


Sodium 


Needed for muscles and 
nerves to work normally 


Table salt, most packaged 
foods 


1.5g 



Your body cannot produce any of the minerals that it needs. Instead, you must get minerals from the foods 
you eat. Good sources of minerals are listed in Table 2. They include milk, green leafy vegetables, and 
whole grains. 

Not getting enough minerals can cause health problems. For example, too little calcium may cause osteo- 
porosis. This is a disease in which bones become soft and break easily. Getting too much of some minerals 
can also cause health problems. Many people get too much sodium. Sodium is added to most packaged 
foods. People often add more sodium to their food by using table salt (sodium chloride). Too much sodium 



423 



causes high blood pressure in some people. 

Water 

Did you know that water is also a nutrient? By weight, your cells are about two-thirds water, so you cannot 
live without it. In fact, you can survive for only a few days without water. You lose water in each breath you 
exhale. You also lose water in sweat and urine. If you do not take in enough water to replace the water that 
you lose, you may develop dehydration. Symptoms of dehydration include dry mouth, headaches, and 
feeling dizzy. Dehydration can be very serious. Severe dehydration can even cause death. 

When you exercise, especially on a hot day, you lose more water in sweat than you usually do. You need 
to drink extra water before, during, and after exercise. The children in Figure 5 are drinking water while 
playing outside on a warm day. They need to drink water to avoid dehydration. 




Figure 5: When you are active outside on a warm day, it's important to drink plenty of water. You need to 
replace the water you lose in sweat. 

(Source: http://www.water.ca.gov/swp/images/geography/Kids.jpg, License: Public Domain) 

Getting too much water can also be dangerous. Excessive water may cause a condition called hyponatremia. 
In this condition, water collects in the brain and causes it to swell. Hyponatremia can cause death. It requires 
emergency medical care. 

Lesson Summary 

• The body needs food for energy, building materials, and substances that help control body processes. 

• Carbohydrates, proteins, and lipids provide energy and have other important roles in the body. 

• Vitamins and minerals do not provide energy but are needed in small amounts for the body to function 
properly. 

• The body must have water to survive. 
Review Questions 

1 . What are three reasons that your body needs food? (Intermediate) 

2. Which nutrients can be used for energy? (Beginning) 



424 



3. Name two types of fiber and state the role of each type of fiber in the body. (Intermediate) 

4. What are some foods that are good sources of vitamin C? (Beginning) 

5. What are two minerals that are needed for strong bones and teeth? (Beginning) 

6. List some of the functions of proteins in the body. Based on your list, predict health problems people might 
have if they do not get enough proteins in foods. (Challenging) 

7. Your body needs 20 different amino acids. Why do you need to get only ten of these amino acids from 
food? Name foods you can eat to get these ten amino acids. (Intermediate) 

8. Compare and contrast saturated and unsaturated lipids. (Intermediate) 

9. Identify three vitamins that are produced in the body. How are they produced? (Intermediate) 

1 0. Why do you need to drink extra water when you exercise on a hot day? What might happen if you did 
not drink extra water? (Intermediate) 

Further Reading I Supplemental Links 

Alexandra Powe Allred. Nutrition. Perfection Learning, 2005. 

Ann Douglas and Julie Douglas. Body Talk: The Straight Facts on Fitness, Nutrition, and Feeling Great 
about Yourself! Maple Tree Press, 2006. 

DK Publishing. Food. DK Children, 2005. 

Donna Shryer. Body Fuel: A Guide to Good Nutrition. Marshall Cavendish Children's Books, 2007. 

Linda Bickerstaff. Nutrition Sense. Rosen Central, 2008. 

CK-12. High School Biology. Chapter 38, Lesson 1. 

http://www.iom.edU/Object.File/Master/21/372/0.pdf 

http://www.nlm.nih.gov/medlineplus/ency/article/002404.htm 

http://www.textbookofbacteriology.net/normalflora.html 

http://en.wikipedia.org/wiki/Vitamins 

Vocabulary 

calories Units used to measure the energy in food. 

carbohydrates Nutrients that include sugars, starches, and fiber; give your body energy; organic 

compound. 

essential amino acids Amino acids that must come from the proteins in foods; you cannot make these 

amino acids. 

insoluble fiber Large, complex carbohydrate; does not dissolve in water; moves through the large 

intestine and helps keep food waste moist so it can pass easily out of the body. 

lipids Nutrients such as fats that are rich in energy; organic compound. 

minerals Chemical elements that are needed for body processes. 

nutrients Chemicals in food that your body needs. 

proteins Nutrients made up of smaller molecules called amino acids; give your body energy; 

help control body processes; organic compound. 



425 



saturated fats Found mainly in animal foods, such as meats, whole milk, and eggs; increase 

cholesterol levels in the blood. 

soluble fiber Large, complex carbohydrate; dissolves in water; helps keep sugar and fat at 

normal levels in the blood. 

starch Large, complex carbohydrate; found in foods such as vegetables and grains; 

broken down by the body into sugars that provide energy. 

trans fat Manufactured and added to certain foods to keep them fresher for longer. Foods 

that contain trans fats include cakes, cookies, fried foods, and margarine. 

unsaturated lipids Found mainly in plant foods, such as vegetable oil, olive oil, and nuts; also found 

in fish such as salmon. 

vitamins Substances that the body needs in small amounts to function properly. 

Review Answers 

1 . Your body needs food for energy, building materials, and substances that help control body processes. 

2. Nutrients that can be used for energy are carbohydrates, proteins, and lipids. 

3. Two types of fiber are soluble fiber and insoluble fiber. Soluble fiber helps keep sugar and fat at normal 
levels in the blood. Insoluble fiber helps keep food waste moist so it can pass easily out of the body. 

4. Some foods that are good sources of vitamin C are oranges, grapefruits, red peppers, and broccoli. 

5. Two minerals that are needed for strong bones and teeth are calcium and phosphorus. 

6. Sample answer: Some of the functions of proteins in the body are providing energy, making up muscles, 
and helping the body fight off bacteria. If people do not get enough proteins in foods, they might have too 
little energy, weak muscles, and reduced ability to fight off bacteria. 

7. You need to get only ten amino acids from foods because the body can make the other ten amino acids 
it requires. Foods you can eat to get the ten amino acids you need include animal foods, such as milk and 
meat. Plant foods are missing one or more of these amino acids. However, by eating a combination of plant 
foods, such as beans and rice, you can get all ten. 

8. Saturated lipids are harmful even in very small amounts. They are found mainly in animal foods, such as 
meats, whole milk, and eggs. Unsaturated lipids are needed in small amounts for good health. They are 
found mainly in plant foods, such as vegetable oil, olive oil, and nuts. 

9. Three vitamins that are produced in the body are vitamins D, B 12 , and K. Vitamin D is made in the skin 
when it is exposed to sunlight. Vitamins B 12 and K are produced by bacteria that normally live inside the 
body. 

10. You need to drink extra water when you exercise on a hot day because you lose more water in sweat 
than you usually do. If you did not drink extra water, you might develop dehydration. Symptoms of dehydration 
include dry mouth, headaches, and feeling dizzy. Dehydration can be very serious. It can even cause death. 

Points to Consider 

• Think about how you can be sure you are getting enough nutrients? 

• Do you think knowing the nutrients in the foods you eat are important? 

• Do you have to keep track of all the nutrients you eat, or is there an easier way to choose foods that 
provide the nutrients you need? 



426 



Choosing Healthy Foods 

Lesson Objectives 

• State how to use MyPyramid to get the proper balance of nutrients. 

• Describe how to read food labels to choose foods wisely. 

• Explain how to balance food with exercise. 

Check Your Understanding 

What is a nutrient? 

Why do you need extra energy when you exercise? 

introduction 

Foods such as whole grain breads, fresh fruits, and fish provide nutrients you need for good health. However, 
various foods provide different nutrients. You also need different amounts of each nutrient. How can you 
choose the right mix of foods to get the proper balance of nutrients? Two tools can help you choose foods 
wisely: MyPyramid and food labels. 

MyPyramid 

MyPyramid is a diagram that shows how much you should eat each day of foods from six different food 
groups. It recommends the amount of nutrients you need based on your age, your sex, and your levels of 
activity. MyPyramid is shown in Figure 1. The six food groups in MyPyramid are: 

Grains — such as bread, rice, pasta, and cereal. 

Vegetables — such as spinach, broccoli, carrots, and sweet potatoes. 

Fruits — such as oranges, apples, bananas, and strawberries. 

Oils — such as vegetable oil, canola oil, olive oil, and peanut oil. 

Milk — such as milk, yogurt, cottage cheese, and other cheeses. 

Meat and beans — such as chicken, fish, soybeans, and kidney beans. 




Orange— Grains 

Green — Vegetables 

Red— Fruits 

Yellow— Oils 

Blue— Milk 

Purple — Meat and Beans 



MyPyramid.gov 

STEPS TO A HEALTHIER yOU 



Figure 1: MyPyramid can help you choose foods wisely for good health. Each colored band represents a 
different food group. The key shows which food group each color represents. Which colored band of 
MyPyramid is widest? Which food group does it represent? 

(Source: http://www.mypyramid.gov/, License: Public Domain) 



427 



Using MyPyramid 

In MyPyramid, each food group is represented by a band of a different color. For example, grains are repre- 
sented by an orange band, and vegetables are represented by a green band. The wider the band, the more 
foods you should choose from that food group each day. The orange band in MyPyramid is the widest band. 
This means that you should choose more foods from the grain group than from any other single food group. 
The green, blue, and red bands are also relatively wide. Therefore, you should choose plenty of foods from 
the vegetable, milk, and fruit groups, as well. You should choose the fewest foods from the food group with 
the narrowest band. Which band is narrowest? Which food group does it represent? 

Healthy Eating Guidelines 

Did you ever hear the saying, "variety is the spice of life"? Variety is also the basis of a healthy eating plan. 
When you choose foods based on MyPyramid, you should choose a variety of different foods. Follow these 
guidelines to make the wisest food choices for good health. Keep in mind that nutritional guidelines may 
change throughout life. As food provides energy and nutrients for growth and development, nutritional re- 
quirements may vary with body weight, age, sex, activity, and body functioning. 

• Make at least half your daily grain choices whole grains. Examples of whole grains are whole wheat 
bread, whole wheat pasta, and brown rice. 

• Choose a variety of different vegetables each day. Be sure to include both dark green vegetables, such 
as spinach and broccoli, and orange vegetables, such as carrots and sweet potatoes. 

• Choose a variety of different fruits each day. Select mainly fresh fruits rather than canned fruits and 
whole fruits instead of fruit juices. 

• When choosing oils, go for unsaturated oils, such as olive oil, canola oil, or vegetable oil. 

• Choose low-fat or fat-free milk and other dairy products. For example, select fat-free yogurt and low-fat 
cheese. 

• For meats, choose fish, chicken, and lean cuts of beef. Also, be sure to include beans, nuts, and seeds. 
What about Ice Cream, Cookies, and Potato Chips? 

Are you wondering where foods like ice cream, cookies, and potato chips fit into MyPyramid? The white tip 
of MyPyramid represents foods such as these. These are foods that should be eaten only in very small 
amounts and not very often. Such foods contain very few nutrients, and are called nutrient-poor. Instead, 
they are high in fats, sugars, and sodium, but low in other nutrients. Fats, sugars, and sodium are nutrients 
that you should limit in a healthy eating plan. Ice cream, cookies, and potato chips are also high in Calories. 
Eating too much of them may lead to unhealthy weight gain. 

Food Labels 

In the United States, packaged foods are required by law to have nutrition facts labels. A nutrition facts 
label shows the nutrients in a food. Packaged foods are also required to list their ingredients. An ingredient 
is a specific item that a food contains. 

Using Nutrition Facts Labels 

An example of a nutrition facts label is shown in Figure 2. The information listed at the right of the label tells 
you what to look for. At the top of the label, look for the serving size. The serving size tells you how much 
of the food you should eat to get the nutrients listed on the label. A cup of food from the label in Figure 2 is 
a serving. The Calories in one serving are listed next. In this food, there are 250 Calories per serving. 



428 



Nutrition Facts 

Serving Size 1 cup (228g) 
Servings Per Container 2 


Start here 


Amount Par serving 


Check calories 


Calories 250 Calories from Fat 11 






% Daily 


Value' 


Quick guide to % DV 


Total Fat I2g 




18% 


5% or less is low 
20% or more is high 


Saturated Fat 3g 




15% 


Trans Fat 3g 


Cholesterol 30 ny 




10% 




Sodium 470mg 




20% 


Limit these 


Potassium 700mg 




20% 




Total Carbohydrate 31 g 




10% 


Gel enough of these 


Dietary Fiber Og 




W7o 


^^^^^^^^^^^^^^ 


Sugars 5g 








Protein 5y 




Vitamin A 




4% 




Vitamin C 




2% 




Calcium 




ao% 




Iron 




4% 


Footnote 


1 Percent Daily Values are based on a 2,000 calorie dieL 
Vour Daily Va ! ues may be higheror lower depending on 
your calorie needs. 

Calories 2.000 2,500 




T0I3I Fat Leas tnan 
Sal Fai Less tnan 
Cholesterol Less tnan 
Sodium Less tnan 
Tuial Carbohydrate 

Di-.lH 1 / Fiber 


65g 
20g 

300mg 
2 40Omg 
300g 
250 


aog 

25rj 

300 mg 
2.40orr!g 
375g 
30g 





Figure 2: Reading nutrition facts labels can help you choose healthy foods. Look at the nutrition facts label 
shown here. Do you think this food is a good choice for a healthy eating plan? Why or why not? 

(Source: http://www.health.gov/dietaryguidelines/dga2005/healthieryou/images/img_tips_food_label.gif, Li- 
cense: Public Domain) 

Next on the nutrition facts label, look for the percent daily values (% DV) of nutrients. A food is low in a nu- 
trient if the percent daily value of the nutrient is 5% or less. The healthiest foods are low in nutrients such 
as fats and sodium. A food is high in a nutrient if the percent daily value of the nutrient is 20% or more. The 
healthiest foods are high in nutrients such as fiber and proteins. Look at the percent daily values on the food 
label in Figure 2. Which nutrients have values of 5% or less? These are the nutrients that are low in this 
food. They include fiber, vitamin A, vitamin C, and iron. Which nutrients have values of 20% or more? These 
are the nutrients that are high in this food. They include sodium, potassium, and calcium. 

Using Ingredients Lists 

The food label in Figure 3 includes the list of ingredients in a different food. The ingredients on food labels 
are always listed in descending order. This means that the main ingredient is listed first. The main ingredient 
is the ingredient that is present in the food in the greatest amount. As you go down the list, the ingredients 
are present in smaller and smaller amounts. 



429 



Nutrition Facts 

Serving SizeVi cup (52 g) 

Servings Per Container 8 



Amount Per Serving 
Calories 200 



Calories from Fat 45 



Total Fat 5 g 

Saturated Fat 2.5 g 
Trans fat g 
Cholesterol mg 
Sodium 160 mg 
Total Carbohydrate 37 g 
Dietary Fiber 1 g 
Sugars 17 g 

Protein 2 g 



% Daily Value* 
8% 
13% 

0% 

7% 

12% 

4% 



Vitamin A 

Iron 

Niacin 



%Vitamin C 
10 %Thiamin 
20 %Vitamin B, 



%Calcium % 
10%Riboflavin 0% 
0%FolicAcid 10% 



'Percent Daily Values are based on a 2000 Calorie diet. Your daily 
values may be higher or lower depending on your calorie needs. 

Ingredients: Enriched wheat flour (wheat 
flour, iron, Vitamin B,, folic acid), high- 
fructose corn syrup, vegetable oil (canola 
and soybean oil, partially hydrogenated 
palm kernel oil), sugar, salt, raisins, 
cornstarch, whole grain oats, baking soda, 
artificial flavor, caramel color 



Ingredients List 



Figure 3: This food label includes the list of ingredients in the food. The main ingredient is enriched wheat 
flour, followed by high-fructose corn syrup. Why should you avoid foods with ingredients such as these at 
the top of the ingredients list? 

(Source: Created by: Jean Brainard) 

Reading the ingredients lists on food labels can help you choose the healthiest foods. At the top of the list, 
look for ingredients such as whole grains, vegetables, milk, and fruits. These are the ingredients you need 
in the greatest amounts for balanced eating. Avoid foods that list fats, oils, sugar, or salt at the top of the 
list. For good health, you should avoid getting too much of these ingredients. Be aware that ingredients such 
as corn syrup are sugars. 

You should also use moderation when eating foods that contain ingredients such as white flour or white 
rice. These ingredients have been processed, and processing removes nutrients. The word "enriched" is a 
clue that an ingredient has been processed. Ingredients are enriched with added nutrients to replace those 
lost during processing. However, enriched ingredients are still likely to have fewer nutrients than unprocessed 
ingredients. 

Balancing Food with Exercise 

Look at MyPyramid in Figure 1. Note the person walking up the side of the pyramid. This shows that exercise 
is important for balanced eating. Exercise helps you use any extra energy in the foods you eat. The more 
active you are, the more energy you use. You should try to get at least an hour of physical activity just about 
every day. Figure 4 shows some activities that can help you use extra energy. 



430 



Balancing Food with Exercise 




Figure 4: All of these activities are good ways to exercise and use extra energy. The Calories given for each 
activity are the number of Calories used in an hour by a person that weighs 100 pounds. Which of these 
activities uses the most Calories? Which of the activities do you enjoy? 

(Sources: basketball: http://www.brooklineadulted.org/images/smartsummers/Basketball.jpg, License: GNU 
Free Documentation; http://www.hud.gov/local/ny/images/hgv-picw-ny-2005-06-27.jpg, License: Public Do- 
main; http://commons.wikimedia.Org/wiki/lmage:Walking.jpg, License: Creative Commons Attribution 
Sharealike 2.0; http://c0mm0ns.wikimedia.0rg/wiki/lmage:Y0uth-s0ccer-indiana.jpg, License: Public Domain; 
http://whatscookingamerica.net/lnformation/CalorieBurnChart.htm, 
http://www.cdc.gov/nchs/data/nhanes/growthcharts/set1clinical/cj 41 I022.pdf, 
http://www.cdc.gov/nchs/data/nhanes/growthcharts/set2/chart%2003.pdf, http://www.self.com/health/activ- 
ity/calculators/soccer) 

Any unused energy in food is stored in the body as fat. This is true whether the extra energy comes from 
carbohydrates, proteins, or lipids. What happens if you take in more energy than you use, day after day? 
You will store more and more fat and become overweight. Eventually, you may become obese. Obesity is 
having a very high percentage of body fat. Obese people are at least 20 percent heavier than their healthy 
weight range. The excess body fat of obesity is linked to many diseases. Obese people often have serious 
health problems, such as diabetes, high blood pressure, and high cholesterol. They are also more likely to 
develop arthritis and some types of cancer. People that remain obese throughout adulthood usually do not 
live as long as people that stay within a healthy weight range. 

The current generation of children and teens is the first generation in our 
history that may have a shorter life than their parents . The reason is their 
high rate of obesity and the health problems associated with obesity. 



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You can avoid gaining weight and becoming obese. The choice is yours. Choose healthy foods by using 
MyPyramid and reading food labels. Then get plenty of exercise to balance the energy in the foods you eat. 

Lesson Summary 

• MyPyramid shows how much you should eat each day of foods from six different food groups. 

• Reading food labels can help you choose the healthiest foods. 

• Regular exercise helps you use extra energy and avoid unhealthy weight gain. 

Review Questions 

1. List the six food groups represented by MyPyramid. (Beginning) 

2. Which food group contains soybeans, kidney beans, and fish? (Beginning) 

3. What guideline should you follow in choosing foods from the grains food group? (Intermediate) 

4. Which ingredient is always listed first on a food label? (Intermediate) 

5. What happens if you take in more energy than you use, day after day? (Intermediate) 

6. Explain how you can use MyPyramid to choose foods that provide the proper balance of nutrients. (Inter- 
mediate) 

7. Why should you limit foods like ice cream and potato chips in a healthy eating plan? (Intermediate) 

8. Explain how you can use food labels to choose foods that are high in fiber. (Intermediate) 

9. Why should you try to avoid foods with processed ingredients? What are some examples of processed 
ingredients? (Challenging) 

10. How does physical activity help prevent obesity? (Intermediate) 

Further Reading I Supplemental Links 

Eric Schlosser and Charles Wilson. Chew on This: Everything You Don't Want to Know about Fast Food. 
Houghton Mifflin, 2006. 

John Burstein. The Shape of Good Nutrition: The Food Pyramid. Crabtree Publishing Company, 2008. 

Rose McCarthy. Food Labels: Using Nutrition Information to Create a Healthy Diet. Rosen Publishing 
Group, 2008. 

Sandra Giddens. Making Smart Choices about Food, Nutrition, and Lifestyle. Rosen Central, 2008. 

CK-12. High School Biology. Chapter 38, Lesson 1. 

http://www.cfsan.fda.gov/~acrobat/nutfacts.pdf 

http://www.cfsan.fda.gov/~dms/foodlab.html 

http://www.fns.usda.gov/tn/parents/nutritionlabel.html 

http://www.health.gov/dietaryguidelines/dga2005/document/pdf/DGA2005.pdf 

http://www.iom.edU/Object.File/Master/21/372/0.pdf 

http:www.mypyramid.gov 



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http://www.newswise.com/articles/view/537296 

http://www.nlm.nih.gov/medlineplus/ency/article/002459.htm 

http://www.nlm.nih.gov/medlineplus/exerciseforchildren.html 

http://www.prb.org/Articles/2005/WillRisingChildhoodObesityDecreaseUSLifeExpectancy.aspx 

http://www.sciencemag.org/cgi/content/summary/307/571 6/1 71 6 

http://en.wikipedia.org/wiki 

Vocabulary 

enriched Term used for an ingredient that has been processed; ingredients are enriched with 

added nutrients to replace those lost during processing; likely to have fewer nutrients 
than unprocessed ingredients. 

ingredient A specific item that a food contains. 

main ingredient The ingredient that is present in the food in the greatest amount. 

MyPyramid Diagram that shows how much you should eat each day of foods from six different 

food groups. 

nutrition facts label The label on packaged food that shows the nutrients in the food. 

obesity Having a very high percentage of body fat; obese people are at least 20 percent 

heavier than their healthy weight range. 

serving size Tells you how much of the food you should eat to get the nutrients listed on the label. 

Review Answers 

1. The six food groups represented by MyPyramid are grains, vegetables, fruits, oils, milk, and meat and 
beans. 

2. The meat and beans food group contains soybeans, kidney beans, and fish. 

3. When choosing foods from the grains food group, you should make at least half your daily choices whole 
grains. 

4. The main ingredient is always listed first on a food label. This is the ingredient that is present in the 
greatest amount. 

5. If you take in more energy than you use, day after day, you will store more and more fat and become 
overweight. Eventually, you may become obese. 

6. You can use MyPyramid to choose foods that provide the proper balance of nutrients by choosing more 
foods each day from the food groups represented by wider bands in MyPyramid. For example, the orange 
band is wider than the other bands. This means that you should eat more grains than otherfoods. The green, 
blue, and red bands are also relatively wide, so you should eat plenty of vegetables, milk products, and 
fruits, as well. You should eat the fewest foods from the group with the narrowest band, which is the oils 
food group. 

7. You should limit foods like ice cream and potato chips in a healthy eating plan because such foods contain 
very few of the nutrients that you need in the greatest amounts. Instead, they are high in nutrients — including 
fats, sugars, and sodium — that you should limit in a healthy eating plan. These foods are also high in Calories, 
so eating too much of them may lead to unhealthy weight gain. 

8. You can use food labels to choose foods that are high in fiber by looking for foods that contain 20 percent 
or more of the percent daily value of fiber. Foods that contain 20 percent or more of the percent daily value 
of a nutrient are considered to be high in that nutrient. 



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9. You should try to avoid foods with processed ingredients because processing removes nutrients. Examples 
of processed ingredients are white flour and white rice. Ingredients that are enriched, such as enriched flour, 
have also been processed. Ingredients are enriched with added nutrients to replace those lost during pro- 
cessing. However, they are still likely to have fewer nutrients than unprocessed ingredients. 

1 0. Obese people are at least 20 percent heavier than their healthy weight. They gain weight because they 
consistently take in more energy than they use and store it as fat. Physical activity helps prevent obesity by 
using the extra energy in food. The more active people are, the more energy they use. 

Points to Consider 

• Discuss how foods may be broken down into nutrients that your body can use? For example, how do 
you think an apple becomes simple sugars that your body can use for energy? Or how might a piece of 
cheese become proteins that your body can use for building materials? 



Digestive System 

Lesson Objectives 

State the functions of the digestive system. 

Explain the role of enzymes in digestion. 

Describe the digestive organs and their functions. 

Explain the roles of helpful bacteria in the digestive system. 

List ways to help keep your digestive system healthy. 

Check Your Understanding 

What is a chemical reaction? 

What is an enzyme? 

What are bacteria? 

introduction 

Nutrients in the foods you eat are needed by the cells of your body. How do the nutrients in foods get to 
your body cells? What organs and processes break down the foods and make the nutrients available to 
cells? The organs are those of the digestive system. The processes are digestion and absorption. 

What Does the Digestive System Do? 

The digestive system is the body system that breaks down food and absorbs nutrients. It also gets rid of 
solid food waste. The main organs of the digestive system are shown in Figure 1. 



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Mouth 



Large intestine - 



Esophagus 




Stomach 



Figure 1 : This drawing shows the major organs of the digestive system. Trace the path of food through the 
organs of the digestive system as you read about them in this lesson. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:Digestivetract.gif, License: Public Domain) 

Digestion is the process of breaking down food into nutrients. There are two types of digestion: mechanical 
digestion and chemical digestion. In mechanical digestion, large chunks of food are broken down into small 
pieces. This is a physical process. In chemical digestion, large food molecules are broken down into small 
nutrient molecules. This is a chemical process. 

Absorption is the process in which substances are taken up by the blood. After food is broken down into 
small nutrient molecules, the molecules are absorbed by the blood. Then the nutrient molecules travel in 
the bloodstream to cells throughout the body. 

Some substances in food cannot be broken down into nutrients. They remain behind in the digestive system 
after the nutrients are absorbed. Any substances in food that cannot be digested and absorbed pass out of 
the body as solid waste. The process of passing solid food waste out of the body is called elimination. 

The Role of Enzymes in Digestion 

Chemical digestion could not take place without the help of digestive enzymes. An enzyme is a protein that 
speeds up chemical reactions in the body. Digestive enzymes speed up chemical reactions that break down 
large food molecules into small nutrient molecules. 

Did you ever use a wrench, like the one in Figure 2, to tighten a bolt? You could tighten a bolt with your 
fingers, but it would be difficult and slow. If you use a wrench, you can tighten a bolt much more easily and 
quickly. Enzymes are like wrenches. They make it much easier and quicker for chemical reactions to take 
place. Like a wrench, enzymes can also be used over and over again. But you need the appropriate size 
and shape of the wrench to efficiently tighten the bolt, just like each enzyme is specific for the reaction it 
helps. 



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$ 



Figure 2: Turning a bolt with a wrench is easier and quicker than trying to turn a bolt with your fingers. How 
is a wrench like an enzyme? 

(Source: http://images1.comstock.com/lmagewarehouse/RF/SITECS/NLWMCompingVersions/0003000/3500- 
3999/KS351 6.jpg, License: GNU Free Documentation) 

Digestive enzymes are secreted by the organs of the digestive system. Examples of digestive enzymes are: 

Amylase is produced by the mouth. It helps break down large starches molecules into smaller sugar 
molecules. 

Pepsin is produced by the stomach. Pepsin is a protease; it helps break down proteins into amino acids. 

Trypsin is produced in the pancreas. Trypsin is a protease; it cleaves peptide chains. 

Pancreatic lipase is secreted by the pancreas. It is a lipase, used to break apart fats. 

Deoxyribonuclease and ribonuclease are nucleases secreted by the pancreas. They are enzymes that 
break bonds in nucleic acid backbones. 

Bile salts are bile acids whose main function is to facilitate the processing of dietary fat. Bile acids are made 
in the liver. Upon eating a meal, the contents of the gallbladder are secreted into the intestine, where bile 
acids break down dietary fats. Bile acids serve other functions, including eliminating cholesterol from the 
body. 

Digestive Organs and Their Roles 

The mouth and stomach are just two of the organs of the digestive system. Other digestive system organs 
are the esophagus, small intestine, and large intestine. From Figure 1 , you can see that the digestive organs 
form a long tube. In adults, this tube is about 9 meters (30 feet) long! At one end of the tube is the mouth. 
At the other end is the anus. Food enters the mouth and then passes through the rest of the digestive system. 
Food waste leaves the body through the anus. 

The organs of the digestive system are lined with muscles. The muscles contract, or tighten, to push food 
through the system. This is shown in Figure 3. The muscles contract in waves. The waves pass through 
the digestive system like waves through a Slinky®. This movement of muscle contractions is called peristalsis. 
Without peristalsis, food would not be able to move through the digestive system. Peristalsis is an involuntary 
process, which means that it occurs without your conscious control. 



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muscles contract 

food 

muscles relax 




direction of movement 



Figure 3: This diagram shows how muscles push food through the digestive system. Muscle contractions 
travel through the system in waves, pushing the food ahead of them. This is called peristalsis. 

(Source: http://upload.wikimedia.Org/wikibooks/en/c/c4/Anatomy_and_physiology_of_animals_Peristalis.jpg, 
License: Creative Commons Attribution 3.0) 

The liver, gall bladder, and pancreas are also organs of the digestive system. They are shown in Figure 4. 
Food does not pass through these three organs. However, these organs are important for digestion. They 
secrete or store enzymes or other chemicals that are needed to help digest food chemically. 




KEY: 




1. 


esophagus 


2. 


stomach 


3. 


liver 


4. 


gall bladder 


5. 


pancreas 


6. 


small intes 



Figure 4: This drawing shows the liver, gall bladder, and pancreas. These organs are part of the digestive 
system. Food does not pass through them, but they secrete substances needed for chemical digestion. 

(Source: http://en.wikipedia.0rg/wiki/lmage:BauchOrgane_wn.png, License: GNU Free Documentation) 



437 



Mouth, Esophagus, and Stomach 

The mouth is the first organ that food enters. However, digestion may start even before you put the first bite 
of food into your mouth. Just seeing or smelling food can cause the release of saliva and digestive enzymes 
in your mouth. Once you start eating, saliva wets the food, which makes it easier to break up and swallow. 
Digestive enzymes, including amylase, start breaking down starches into sugars. Your tongue helps mix 
the food with the saliva and enzymes. 

Your teeth also help digest food. Your front teeth are sharp. They cut and tear food when you bite into it. 
Your back teeth are broad and flat. They grind food into smaller pieces when you chew. Chewing is part of 
mechanical digestion. Your tongue pushes the food to the back of your mouth so you can swallow it. When 
you swallow, the lump of chewed food passes down your throat to your esophagus. 

The esophagus is a narrow tube that carries food from the throat to the stomach. Food moves through the 
esophagus because of peristalsis. At the lower end of the esophagus, a circular muscle controls the opening 
to the stomach. The muscle relaxes to let food pass into the stomach. Then the muscle contracts again to 
prevent food from passing back into the esophagus. 

Some people think that gravity moves food through the esophagus. If that 
were true, food would move through the esophagus only when you are sitting 
or standing upright. In fact, because of peristalsis, food can move through 
the esophagus no matter what position you are in— even upside down. Just don't 
try to swallow food when you upside down! You could choke if you try to 
swallow when you are not upright. 



The stomach is a sac-like organ at the end of the esophagus. It has thick muscular walls. The muscles al- 
ternately contract and relax. This churns the food and helps break it into smaller pieces. The churning also 
mixes the food with the enzyme pepsin and other chemicals that are secreted by the stomach. The pepsin 
and other chemicals help digest proteins chemically. 

Water, salt, and simple sugars can be absorbed into the blood from the stomach. Most other substances 
are broken down further in the small intestine before they are absorbed. The stomach stores food until the 
small intestine is ready to receive it. A circular muscle controls the opening between the stomach and small 
intestine. When the small intestine is empty, the muscle relaxes. This lets food pass from the stomach into 
the small intestine. 

Small Intestine 

The small intestine is narrow tube that starts at the stomach and ends at the large intestine (see Figure 
1). In adults, the small intestine is about 7 meters (23 feet) long. It is made up of three parts: the duodenum, 
jejunum, and ileum. Each part has different functions. 

The duodenum is the first part of the small intestine. This is where most chemical digestion takes place. 
Many enzymes and other chemicals are secreted here. Some are secreted by the duodenum itself. Others 
are secreted by the pancreas or liver. 

The jejunum is the second part of the small intestine. This is where most nutrients are absorbed into the 
blood. The jejunum is lined with tiny "fingers" called villi. A magnified picture of villi is shown in Figure 5. 
Villi contain microscopic blood vessels. Nutrients are absorbed into the blood through these tiny vessels. 
There are millions of villi, so altogether there is a very large area for absorption to take place. In fact, villi 
make the inner surface area of the small intestine 1,000 times larger than it would be without them. The 
entire inner surface area of the small intestine is about as big as a basketball court! 



438 




Figure 5: This is what the villi lining the small intestine look like when magnified. Each one is actually only 
about 1 millimeter long. Villi are just barely visible with the unaided eye. 

(Source: http://kalishresearch.com/images/villi.jpg, License: GNU Free Documentation) 

The ileum is the third part of the small intestine. Like the jejunum, the ileum is covered with villi. A few re- 
maining nutrients are absorbed in the ileum. From the ileum, any remaining food waste passes into the large 
intestine. 

The small intestine is much longer than the large intestine. So why is it 
called "small"? If you compare the small and large intestines in Figure 1 , 
you will see why. The small intestine is smaller in width that the large 
intestine. 



Large Intestine 

The large intestine is a relatively wide tube that connects the small intestine with the anus. In adults, it is 
about 1 .5 meters (5 feet) long. Waste enters the large intestine from the small intestine in a liquid state. As 
the waste moves through the large intestine, excess water is absorbed from it. After the excess water is 
absorbed, the remaining solid waste is called feces. Circular muscles control the anus. They relax to let the 
feces pass out of the body through the anus. After feces pass out of the body, they are called stool. The 
excretion of stool is referred to as a bowel movement. 

Liver 

The liver has a wide range of functions, a few of which are blood detoxification, maintaining glucose balance, 
protein synthesis, and production of biochemicals necessary for digestion. The liver is necessary for survival; 
there is currently no way to compensate for the absence of liver function. 

The liver is one of the most important organs in the body when it comes to detoxifying or getting rid of foreign 
substances or toxins, especially from the gut. The liver filters blood from the intestine. This filtering process 
can remove a wide range of microorganisms such as bacteria, fungi, viruses and parasites from the blood. 
Almost 2 quarts of blood pass through the liver every minute. 

The liver also has several roles in maintaining glucose levels, including gluconeogenesis (the synthesis of 
glucose from certain amino acids, lactate or glycerol), glycogenosis (the breakdown of glycogen into glu- 
cose), and glycogenosis (the formation of glycogen from glucose). 

Bacteria in the Digestive System 

The large intestine provides a home for trillions of bacteria. Most of these bacteria are helpful. They have 
several roles in the body. For example, intestinal bacteria: 



439 



• Produce vitamins B 12 and K. 

• Control the growth of harmful bacteria. 

• Break down poisons in the large intestine. 

• Break down some substances in food that cannot be digested, such as fiber and some starches and 
sugars. 

Keeping Your Digestive System Healthy 

Most of the time, you probably aren't aware of your digestive system. It works well without causing any 
problems. However, most people have problems with their digestive system at least once in awhile. Did you 
ever eat something that didn't "agree" with you? Maybe you had a stomachache or felt sick to your stomach. 
Maybe you had diarrhea. These could be symptoms of foodborne illness. 

Foodborne Illness 

Harmful bacteria can enter your digestive system in food and make you sick. This is called foodborne illness. 
The bacteria, or the toxins they produce, may cause vomiting or cramping, in addition to the symptoms 
mentioned above. You can help prevent foodborne illness by following a few simple rules: 

• Keep hot foods hot and cold foods cold. This helps prevent any bacteria in the foods from multiplying. 

• Wash your hands before you prepare or eat food. This helps prevent bacteria on your hands from getting 
on the food. 

• Wash your hands after you touch raw foods such as meats, poultry, fish, or eggs. These foods often 
contain bacteria that your hands could transfer to your mouth. 

• Cook meats, poultry, fish, and eggs thoroughly before eating them. The heat of cooking kills any bacteria 
the foods may contain so they cannot make you sick. 

Food Allergies 

Food allergies are like other allergies. They occur when the immune system reacts to harmless substances 
as though they were harmful. Almost 10 percent of children have food allergies. Some of the foods most 
likely to cause allergies are shown in Figure 6. Eating foods you are allergic to may cause vomiting, diarrhea, 
or skin rashes. Some people are very allergic to certain foods. Eating even tiny amounts of the foods causes 
them to have serious symptoms, such as difficulty breathing. If they eat the foods by accident, they may 
need emergency medical treatment. 



440 



Foods that Commonly Cause Allergies 



Nuts 



Fish 




Eggs 



Milk 



Shellfish 



Figure 6: Some of the foods that commonly cause allergies are shown here. They include nuts, eggs, fish, 
milk, and shellfish. Are you allergic to any of these foods? 

(Sources: http://upload.wikimedia.Org/wikipedia/commons/5/57/Mixed_nuts.jpg, License: Public Domain; 
http://upload.wikimedia.0rg/wikipedia/commons/6/6a/Eggs_in_carton.jpg, License: GNU Free Documentation; 
http://commons.wikimedia.0rg/wiki/lmage:Rainbow_Trout.jpg, License: Public Domain; http://commons.wiki- 
media.org/wiki/lmage:Milk_glass.jpg, License: GNU Free Documentation; http://commons.wikime- 
dia.org/wiki/lmage:Garnelen_im_Verkauf_fcm.jpg, License: Public Domain; http://commons.wikime- 
dia.org/wiki/lmage:Shellfish.jpg, License: Creative Commons Attribution 2.5) 

If you think you may have food allergies, a doctor can test you to find out for sure. The tests will identify 
which foods you are allergic to. Then you can avoid eating these foods. This is the best way to prevent the 
symptoms of food allergies. To avoid the foods you are allergic to, you may have to read food labels carefully. 
This is especially likely if you are allergic to common food ingredients, such as soybeans, wheat, or peanuts. 

A food intolerance, or food sensitivity, is different to a food allergy. A food intolerance happens when the 
digestive system is unable to break down a certain type of food. This can result in stomach cramping, diarrhea, 
tiredness, and weight loss. Food intolerances are often mistakenly called allergies. Lactose intolerance is 
a food intolerance. A person who is lactose intolerant does not make enough lactase, the enzyme that breaks 
down the milk sugar lactose. About 75 percent of the world's population is lactose intolerant. 



441 



Constipation 

Constipation means that a person has three bowel movement or less each week. The stools may also be 
hard and dry. Sometimes the stools are difficult or painful to pass. The person may feel "draggy" and full. 

Some people think they should have a bowel movement every day. This is not 
necessarily true. There is no "right" number of bowel movements. What is 
normal for one person may not be normal for another. It depends on the foods 
they eat, how much they exercise, and other factors. 



At one time or another, almost everyone has constipation. In most cases, it lasts for a short time and isn't 
serious. However, constipation can be very uncomfortable. You can follow these tips to help prevent it: 

• Eat enough high-fiber foods, including vegetables, fruits, beans, and whole grains. 

• Drink plenty of water and other liquids. 

• Exercise regularly. 

• Don't ignore the urge to have a bowel movement. 

Following these tips will help keep your digestive system working properly. It will help you feel good and 
stay healthy. 

Lesson Summary 

The digestive system breaks down food, absorbs nutrients, and gets rid of food wastes. 

Digestive enzymes speed up the reactions of chemical digestion. 

The main organs of the digestive system are the mouth, esophagus, stomach, small intestine, and large 
intestine. 

Bacteria in the large intestine produce vitamins and have other roles in the body. 

You can follow simple tips to help keep your digestive system healthy. 

Review Questions 

1 . What are three functions of the digestive system? (Beginning) 

2. Describe the roles of the mouth in digestion. (Intermediate) 

3. In which organs of the digestive system does absorption of nutrients take place? (Intermediate) 

4. Identify two roles of helpful bacteria in the large intestine. (Beginning) 

5. List two rules that can help prevent foodborne illness. (Beginning) 

6. Explain the role of enzymes in digestion. Give examples to illustrate your answer. (Intermediate) 

7. Describe peristalsis, and explain why it is necessary for digestion. (Intermediate) 

8. How can the inner surface area of the small intestine be as big as a basketball court? How does this help 
the small intestine absorb nutrients? (Intermediate) 

9. Assume a person has an illness that prevents the large intestine from doing its normal job. Why might 
the person have diarrhea? (Challenging) 



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1 0. Explain why eating high-fiber foods can help prevent constipation. (Challenging) 
Further Reading I Supplemental Links 

CK-12, High School Biology, Chapter 38, Lesson 2. 

Carol Ballard. The Digestive System. Heinemann Library, 2003. 

Robert J. Sullivan. Digestion and Nutrition. Chelsea House Publications, 2004. 

Sherri Mabry Gordon. Peanut Butter, Milk, and Other Deadly Threats: What You Should Know about Food 
Allergies. Enslow Publishers, 2006. 

Steve Parker. Break It Down: The Digestive System. Raintree, 2006. 

http://digestive.niddk.nih.gov/ddiseases/pubs/bacteria 

http://digestive.niddk.nih.gov/ddiseases/pubs/constipation_ez 

http://hypertextbook.com/facts/2001/AnneMarieThomasino.shtml 

http://kalishresearch.com/a_gluten.html 

http://physiwiki.wetpaint.com/page/Chapter+4:+Enzymes+and+Energy?t=anon 

http://www.biologyinmotion.com/minilec/wrench.html 

http://www.cfsan.fda.gov/~dms/a2z-b.html 

http://www.fsis.usda.gov/Factsheets/Cleanliness_Helps_Prevent_Foodborne_lllness/index.asp 

http://www.mayoclinic.com/health/food-allergies/AA00057 

http://www.textbookofbacteriology.net/normalflora.html 

http://en.wikipedia.org/wiki/Stomach 

Vocabulary 

absorption Process in which substances are taken up by the blood; after food is broken down 

into small nutrient molecules, the molecules are absorbed by the blood. 

chemical digestion Digestion in which large food molecules are broken down into small nutrient 
molecules. 

constipation Having three or less bowel movements each week. 

digestion Process of breaking down food into nutrients. 

digestive system Body system that breaks down food, absorbs nutrients, and gets rid of solid food 

waste. 
duodenum The first part of the small intestine; where most chemical digestion takes place. 

elimination The process in which solid food waste passes out of the body. 

enzyme A substance, usually a protein, that speeds up chemical reactions in the body. 

esophagus The narrow tube that carries food from the throat to the stomach. 

food allergies A condition in which the immune system reacts to harmless substances in food as 

though they were harmful. 

foodborne illness An illness caused by harmful bacteria that enter the digestive system in food. 

food intolerance Occurs when the digestive system is unable to break down a certain type of food. 



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ileum The third part of the small intestine; covered with villi; the few remaining nutrients 

are absorbed in the ileum. 

jejunum The second part of the small intestine; where most nutrients are absorbed into the 

blood; lined with tiny "fingers" called villi. 

large intestine The relatively wide tube between the small intestine and anus where excess water 

is absorbed from food waste. 

mechanical digestion Digestion in which large chunks of food are broken down into small pieces. 

peristalsis Involuntary muscle contractions which push food through the digestive system. 

small intestine The narrow tube between the stomach and large intestine where most chemical 

digestion and absorption of nutrients take place. 

stomach The saclike organ at the end of the esophagus where proteins are digested. 

villi Contain microscopic blood vessels; nutrients are absorbed into the blood through 

these tiny vessels; located on the jejunum and the ileum. 

Review Answers 

1 . Three functions of the digestive system are to break down food, absorb nutrients, and get rid of solid food 
waste. 

2. In the mouth, saliva wets food, making it easier to break up and swallow. Digestive enzymes in the mouth 
start breaking down starches into sugars. The teeth cut and tear food when you bite into it, and they grind 
food into smaller pieces when you chew. The tongue helps mix food with saliva and enzymes and also helps 
you swallow food. 

3. Absorption of nutrients takes place in the stomach and small intestine. Water, salt, and simple sugars 
can be absorbed in the stomach. Most other nutrients are absorbed in the small intestine. 

4. Two roles of helpful bacteria in the large intestine are (any two): producing vitamins B12 and K, controlling 
the growth of harmful bacteria, breaking down poisons in the large intestine, breaking down some substances 
in food that cannot digested. 

5. Two rules that can help prevent foodborne illness are (any two): keep hot foods hot and cold foods cold; 
wash your hands before you prepare or eat food; wash your hands after you touch raw foods such as meats, 
poultry, fish, or eggs; cook meats, poultry, fish, and eggs thoroughly before eating them. 

6. Digestive enzymes speed up the chemical reactions that break down large food molecules into small 
nutrient molecules. Enzymes make it easier and quicker for the chemical reactions to take place. Two exam- 
ples of digestive enzymes are amylase and pepsin. Amylase is produced by the mouth. It helps break down 
starches into sugars. Pepsin is produced by the stomach. It helps break down proteins into amino acids. 

7. Peristalsis is the wavelike movement of muscle contractions through the organs of the digestive system. 
The muscle contractions push food through the system. Peristalsis is necessary for digestion because food 
would not be able to move through the digestive system without it. 

8. The inner surface of the small intestine can be as big as a basketball court because it is lined with tiny 
"fingers" called villi. Because there are millions of villi, the inner surface area of the small intestine is 1,000 
times larger than it would be without them. This helps the small intestine absorb nutrients because villi 
contain microscopic blood vessels. Nutrients are absorbed into the blood through these tiny vessels. 

9. A person with an illness that prevents the large intestine from doing its normal job might have diarrhea 
because the large intestine normally absorbs excess water from food waste. After excess water is removed 
from food waste in the large intestine, it becomes solid feces, which pass from the body in a bowel movement. 
If excess water is not removed from food waste, the feces will contain too much water, and the person might 
have diarrhea. 



444 



10. In constipation, the stool is hard and dry and difficult to pass. Eating high-fiber foods can help prevent 
constipation because soluble fiber in food waste absorbs water as the waste moves through the large intestine. 
This helps keep the food waste moist so it can pass easily out of the body. 

Points to Consider 

• After nutrients are absorbed into the blood, think about how the blood could carry them to all the cells of 
the body. How does the blood travel? What keeps the blood moving? 



445 



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18. Cardiovascular System 



Introduction to the Cardiovascular System 

Lesson Objectives 

• Identify the main structures of the cardiovascular system. 

• Identify three types of blood vessels. 

• Describe the differences between the pulmonary and the systemic circulations. 

• Identify the main structures of the lymphatic system. 

• Outline how the cardiovascular and the lymphatic systems work together. 

Check Your Understanding 

• What is an organ system? 

• What are the three types of muscles found in the human body? 

Introduction 

Your cardiovascular system has many jobs. It acts as a message delivery service, a pump, a heating system, 
and a protector of the body against infection. Every cell in your body depends on your cardiovascular system. 
In this chapter, you will learn how your cardiovascular system works and how it helps to maintain homeostasis. 

Functions of the Cardiovascular System 

The cardiovascular system shown in Figure 1 is the organ system that is made up of the heart, the blood 
vessels, and the blood. Your cardiovascular system has many important roles in maintaining homeostasis. 
It moves nutrients, hormones, gases and wastes to and from your body cells. It also helps to keep you warm 
by moving warm blood around your body. To do these tasks, your cardiovascular system works with other 
body systems such as the respiratory, endocrine, and nervous systems. 



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Figure 1: The cardiovascular system moves nutrients and other substances through cells. 
(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Blutkreislauf.png, License: CC-BY-SA 2.5) 

Parts of the Cardiovascular System 

Your heart pushes the blood around your body through the blood vessels. The heart, shown in Figure 2, is 
made of cardiac muscle (refer to the Skin, Bones, and Muscles chapter). The heart is connected to many 
blood vessels that bring blood all around the body. The cardiac muscle contracts and pumps blood through 
the heart and blood vessels. 




Figure 2: Blood is collected in the heart and pumped out to the lungs, where it releases carbon dioxide and 
picks up oxygen before it is pumped to the rest of the body. 

(Sources: http://commons.wikimedia.0rg/wiki/lmage:Heart_circulation_diagram.svg; http://commons.wikime- 
dia.org/wiki/lmage:Humhrt2.jpg, License: CC-BY-SA 2.5, Photographer: Patrick J. Lynch, Image by: NC 



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Medical Illustrator C. Carl Jaffe, M.D., Photographer: en:User:Stanwhit607) 

Blood Vessels 

The job of these blood vessels is to channel the blood around the body. There are three main types of blood 
vessels in the body; arteries, veins, and capillaries. 

Arteries are blood vessels that carry blood away from the heart. Arteries have thick walls that have a layer 
of smooth muscle, as shown in Figure 3. Arteries usually carry oxygen-rich blood around the body. The 
blood that is in arteries is under pressure. The contractions of the heart muscle causes blood to exert force 
on the walls of the arteries. This force is referred to as blood pressure. Blood pressure is highest in the ar- 
teries and decreases as the blood moves into smaller blood vessels. Thick walls help prevent arteries from 
bursting from the pressure of blood. 

Artery Wall 

Smooth muscle 

Epithelial tissue^--^ /Tunica externa 

basement 
membrane 



Tunica intima 



Figure 3: Arteries are thick-walled vessels with many layers, including a layer of smooth muscle. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/3/32/lllu_artery.jpg, License: Public Domain-gov) 

Every cell in the body needs oxygen, but arteries are too large to bring oxygen and nutrients to single cells. 
Further from the heart, arteries form smaller arteries. These smaller arteries branch into smaller vessels. 
The smaller blood vessels help to bring nutrients and oxygen and take away waste from body tissues. 

The tiniest blood vessels in the body are called capillaries. The walls of capillaries are only a single layer 
of cells thick. Capillaries connect arteries and veins together, as shown in Figure 4. Capillaries also allow 
the delivery of water, oxygen and other substances to body cells. They also collect carbon dioxide and other 
wastes from cells and tissues. Capillaries are so narrow that blood cells must move in single file through 
them. 

A capillary bed is the network of capillaries that supply an organ with blood. The more metabolically active 
a tissue or organ is, the more capillaries it needs to get nutrients and oxygen. 




Capillaries Arteriole Venule 




Artery capillaries Tissue cells 'Vein 



Figure 4: Capillaries connect arteries and veins. 

(Source: http://en.wikipedia.0rg/wiki/lmage:lllu_capillary.jpg, License: Public Domain-gov) 

Blood is carried back to the heart in blood vessels called veins. Veins have thinner walls than arteries do, 
as you can see in Figure 5. The blood in veins is not under pressure. Veins have valves that stop blood from 



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moving backward. Blood is moved forward in veins when the surrounding skeletal muscles squeeze the 
veins. Blood that is carried by veins is usually low in oxygen. The exception is the pulmonary veins that return 
oxygen-rich blood to the heart from the lungs. 



Vein 



Tunica intima 




Tunica externa 



Tunica meida 



Figure 5: The walls of veins are not as thick as artery walls; veins have valves that stop blood from flowing 
backward. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/6/65/lllu_vein.jpg, License: Public Domain-gov) 

Blood is a body fluid that is a type of connective tissue. Blood is made of blood cells, and a fluid called 
plasma. The main types of cells found in blood are red blood cells and white blood cells. Red blood cells 
are the cells that carry oxygen. Oxygen-rich blood is bright red and oxygen-poor blood is dark red. You will 
learn more about blood in a later lesson in this chapter. 

The cardiovascular system of humans is closed. That means the blood never leaves the large loop of blood 
vessels in which it travels. Other animals such as invertebrates have open circulatory systems, in which 
their blood can leave the blood vessels. 

Two Blood Circulation Systems 

The blood is pumped around in two large "loops" within the body. One loop moves blood around the body — to 
the head, limbs, and internal organs. The other loop moves blood to and from the lungs where carbon 
dioxide is released and oxygen is picked up by the blood. Figure 6 is a simple version of these two "loops". 
Systemic circulation is the portion of the cardiovascular system which carries oxygen-rich blood away 
from the heart, to the body, and returns oxygen-poor blood back to the heart. The pulmonary circulation 
is the part of the cardiovascular system which carries oxygen-poor blood away from the heart to the lungs, 
and returns oxygen-rich blood back to the heart. This oxygen-rich blood then gets pumped around the body 
in the systemic circulation. These two circulations will be further discussed in Lesson 2. 



Lungs 



Heart 



Pulmonary circulation 



Systemic circulation 



Body 



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Figure 6: The double circulatory system; blood in one circuit has to go through the heart to enter the other 
circuit. 

(Source: http://upload.wikimedia.0rg/wikipedia/en/f/f6/Double_circulatory_system.jpg, Image by: Joshuajohn- 
lee, License: Public Domain) 

The Lymphatic System 

The lymphatic system is a network of vessels and tissues that carry a clear fluid called lymph. The lymphatic 
system, shown in Figure 7, extends all around the body. Lymph tissues include lymph nodes, lymph ducts, 
and lymph vessels. Lymph vessels are tube-shaped just like blood vessels. The lymphatic system works 
with the cardiovascular system to return body fluids to the blood. The lymphatic system and the cardiovas- 
cular system are often called the body's two circulatory systems. 




Tonsil 



Thvmus Gland 



Spleen 



Lymph Nodes 



Lymphatic Vessels 



Figure 7: The lymphatic system helps return fluid that leaks from the blood vessels back to the cardiovas- 
cular system. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/0/03/lllu_lymphatic_system.jpg, License: Public 
Domain) 

The lymphatic system has two main jobs: 

• Removing excess fluids from body tissues. 



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• Making certain types of white blood cells. 

Role of the Lymphatic System in Circulation 

The lymphatic system collects and returns fluid to the cardiovascular system. A small amount of fluid leaks 
from the blood vessels when blood is pumped around your body. This fluid collects in the spaces between 
cells and tissues. Some of the fluid returns to the cardiovascular system, and the rest is collected by the 
lymph vessels of the lymphatic system, which are shown in Figure 8. 

The fluid that collects in the lymph vessels is called lymph. The lymphatic system then returns the lymph to 
the cardiovascular system. Unlike the blood system, the lymphatic system is not closed and has no central 
pump. Lymph moves slowly in lymph vessels. It is moved along in the lymph vessels by the squeezing action 
of smooth muscles and skeletal muscles. 



Lymph Capillaries in the Tissue Spaces 

Blood capillary - 

Lymph capillary 



Tissue cells 




Tissue spaces 



Venule 



Lymphatic vessel 



Figure 8: Lymph capillaries collect fluid that leaks out from blood capillaries. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/1/19/lllu lymph capillary.png, License: Public Domain) 

Role of the Lymphatic System in the Body's Defenses 

The lymphatic system also plays an important role in the immune system. The lymphatic system makes 
certain blood cells, and also filters, or traps foreign particles. The lymphatic system and contain white blood 
cells to protect the body from infection. 

Organs of the Lymphatic System 

Along with the lymph vessels, lymph ducts, and lymph nodes, the lymphatic system also includes many or- 
gans. The tonsils, thymus, and spleen, which are shown Figure 7, each have a role in the defense of the 
body against infection. Many of these organs are also part of the immune system. 

Tonsils 

The tonsils are areas of lymphoid tissue on either side of the throat. The term tonsils refers most often to 
the tonsils in the back of the throat as shown in Figure 9. But, there are tonsils in the nasal cavity and behind 
the tongue too. Like other organs of the lymphatic system, the tonsils are also part of the immune system. 
The immune system help protect the body against infection. The tonsils are believed to help fight off nose 
and throat and other upper respiratory tract infections such as colds. Tonsils tend to reach their largest size 
near puberty, after which they get smaller. Tonsillitis is an infection of the tonsils that can cause a sore throat 
and fever. 



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Soft palate 



Tonsils 




Tonsils 
Uvula 



Tongue 



Figure 9: The term tonsils refers most often to the tonsils in the back of the throat, but there are tonsils in 
the nasal cavity and behind the tongue too. 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/8/8l/Tonsils_diagram.jpg, License: Public Domain) 

Bone Marrow 

Bone marrow is the tissue found in the middle of bones. The marrow in the large bones of adults makes 
new blood cells. Certain white blood cells, called T-cells, are made in the bone marrow and move to the 
thymus to mature. Other white blood cells called B-cells, move from the bone marrow to the spleen after 
they have matured. 

Thymus 

The thymus is found in the upper chest. Chemicals made by the thymus help the production of certain infec- 
tion-fighting cells. The thymus is where certain white blood cells called lymphocytes mature. These cells 
move from the bone marrow to the thymus to finish growing. The thymus grows to its largest size near puberty, 
and gets smaller as a person ages. If a person's thymus is surgically removed or damaged by disease while 
they are young, the person will be very prone to infections. 

Spleen 

The spleen is in the abdomen, as shown in Figure 10. In an area of the spleen called red pulp, materials 
are filtered from the blood, including old and dead red blood cells. The spleen also makes red blood cells. 
Areas called white pulp help fight infections by making white blood cells. If a person's spleen is surgically 
removed, or does not work properly, the person is prone to certain infections. 

You can learn more about the roles of the lymphatic system and white blood cells in the Diseases and the 
Body's Defenses chapter. 



Table 1: Structures and Functions of the Cardiovascular and Lymphatic Systems 



System 


Structure (organs and tis- 
sues) 


Function 


Lymphatic 


Lymph vessels 


Transports fluid (lymph) from between body cells back 
to blood 




Lymph nodes 


Traps invading microbes, foreign particles, cancerous 
cells 




Spleen, tonsils, and adenoids 


Traps invading microbes and foreign particles 




Thymus 


Site of white blood cell (lymphocyte)maturation 


Cardiovascular 


Blood vessels 


Transports blood around the body 



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Blood 


Transport of oxygen and nutrients; transports white blood 
cells to sites of infection and inflammation 




Heart 


Pumps blood around the body 



Spleen 




White pulp 
Red pulp - 
Capsule - 




Spleen 



Figure 10: In the spleen, the white pulp makes white blood cells, and the red pulp acts like a filter that removes 
dead and dying cells from the blood. 

(Source: http://en.wikipedia.0rg/wiki/lmage:lllu_spleen.jpg, License: Public Domain) 

Lesson Summary 

• The cardiovascular system consists of the heart, the blood vessels, and the blood. There are three main 
types of blood vessels in the body; arteries, veins, and capillaries. 

• The systemic circulation is the portion of the cardiovascular system, which carries oxygen-rich blood 
away from the heart, to the body, and returns oxygen-poor blood back to the heart. The pulmonary circu- 
lation is the part of the cardiovascular system, which carries oxygen-poor blood away from the heart to 
the lungs, and returns oxygen-rich blood back to the heart. 

• Lymph tissues include lymph nodes, lymph ducts, and lymph vessels. Organs of the lymphatic system 
include the tonsils, thymus, and spleen. The lymphatic system works with the cardiovascular system to 
return body fluids to the blood. The two systems together are often called the body's two circulatory 
systems. 

Review Questions 

1 . Identify the main structures of the cardiovascular system. (Beginning) 

2. Identify three types of blood vessels found in the body. (Beginning) 

3. Which blood vessels bring blood away from the heart? (Beginning) 

4. What are the smallest blood vessels in the body called? (Beginning) 

5. What blood vessels bring blood back to the heart? (Beginning) 



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6. Where does blood in the pulmonary system go once it leaves the heart? (Intermediate) 

7. Where does blood in the systemic circulation go once it leaves the heart? (Intermediate) 

8. What does blood that leaves the heart in the systemic circulation have that body cells need? (Intermediate) 

9. Identify the main tissues and organs of the lymphatic system. (Intermediate) 

10. Outline how the cardiovascular and the lymphatic systems work together. (Intermediate) 

11 . What is lymph, and where does it come from? (Intermediate) 

12. Identify one function of tonsils. (Beginning) 

13. What might happen if a person did not have a spleen? (Intermediate) 

14. Name the two circulatory systems of the body. (Intermediate) 
Further Reading I Supplemental Links 
http://en.wikipedia.org/wiki/Heart 

Vocabulary 

arteries Blood vessels that carry blood away from the heart. 

blood A body fluid that is a type of connective tissue; moves oxygen and other com- 

pounds throughout the body. 

capillaries The smallest and narrowest blood vessels in the body. 

cardiovascular system The organ system that is made up of the heart, the blood vessels, and the blood. 

lymphatic system A network of vessels and tissues that carry a clear fluid called lymph; includes 

lymph nodes, lymph ducts, and lymph vessels. 

plasma The straw-colored fluid in blood. 

pulmonary circulation The part of the cardiovascular system which carries oxygen-poor blood away 

from the heart to the lungs, and returns oxygen-rich blood back to the heart. 

systemic circulation The portion of the cardiovascular system which carries oxygen-rich blood away 

from the heart to the body, and returns oxygen-poor blood back to the heart. 

veins Blood vessels that carry blood back to the heart. 

Review Answers 

1. heart, blood vessels, and blood 

2. arteries, veins, and capillaries 

3. arteries 

4. capillaries 

5. veins 

6. blood goes to the lungs 

7. it goes around the body 

8. it has oxygen and nutrients 



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9. lymph tissues include lymph nodes, lymph ducts, and lymph vessels; organs of the lymphatic system include 
the tonsils, thymus, and spleen 

10. the lymphatic system collects and returns fluid to the cardiovascular system 

11 . the fluid that leaks from the blood vessels and that is collected in lymph vessels 

12. they help protect the body against infection 

13. the person would be more likely to get certain infections 

14. the cardiovascular system and the lymphatic system 

Points to Consider 

• Consider how the structure of the heart helps to maintain the systemic and pulmonary circulations. 

• Consider how problems with the coronary circulation can affect the entire body. 

• How would a hole in the heart muscle that allowed blood in the two ventricles to mix affect the rest of 
the body? 

Heart and Blood Vessels 

Lesson Objectives 

• Describe the structure of the heart. 

• Outline how blood moves through the heart. 

• Describe the importance of valves in the heart. 

• Describe the coronary circulation. 

Check Your Understanding 

• What is the role of the cardiovascular system? 

• What is the main function of the heart? 

Introduction 

The heart is divided into four chambers, the left and right atria and the left and right ventricles. An atrium 
is one of the two small, thin-walled chambers on the top of the heart that blood first enters. A ventricle is 
one of the two muscular V-shaped chambers that pump blood out of the heart. The four chambers of the 
heart are shown in Figure 1 . The atria receive the blood, and the ventricles pump the blood out of the heart. 
Each of the four chambers of the heart have a specific job, these are: 

• The right atrium receives oxygen-poor blood from the body. 

• The right ventricle pumps oxygen-poor blood toward the lungs. 

• The left atrium receives oxygen-rich blood from the lungs. 

• The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body. 

The heart is usually found in the left to middle of the chest with the largest part of the heart slightly to the 
left. The heart is usually felt to be on the left side because the left ventricle is bigger and stronger than the 



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right ventricle. The heart is surrounded by the lungs. 



Oxygen-poor 
blood from the 
body returns to 
the heart. 

Right atriu 



Valve 



Right ventricl 




Oxygen-rich blood is pumped 
out of the left ventricle and to 
the rest of the body 

Oxygen-poor blood gets 
pumped to the lungs to 
release carton dioxide 
and pick up oxygen. 



- Left atrium 



Left ventricle 



Figure 1 : The atria receive blood and the ventricles pump blood out of the heart. 

(Source: http://upl0ad.wikimedia.0rg/wikipedia/c0mm0ns/e/ee/A0rta.jpg, Image by: Osnimf, License: Public 
Domain) 

Blood Flow Through the Heart 

Blood flows through the heart in two separate loops; you could think of them as a "left side loop" and a "right 
side loop". The right side and left side of the heart refer to your heart as it sits inside your chest. Its left side 
is your left side and, its right side is your right side. 

The right side of the heart collects deoxygenated blood from the body and pumps it into the lungs where it 
releases carbon dioxide and picks up oxygen. The left-side carries the oxygenated blood back from the 
lungs, into the left side of the heart which then pumps the oxygenated blood throughout the rest of the body. 

The Heartbeat 

To move blood through the heart, the cardiac muscle needs to contract in an organized way. Blood first 
enters the atria, as shown in Figure 2. When the atria contract blood is pushed into the ventricles. After the 
ventricles fill with blood, they contract and blood is pushed out of the heart. Valves in the heart keep the 
blood flowing in one direction. You can see some of the valves in Figure 2. The valves do this by opening 
and closing in one direction only. Blood moves only forward through the heart. The valves stop the blood 
from flowing backward. There are four valves of the heart: 

• The two atrioventricular (AV) valves stop blood from moving from the ventricles to the atria. 

• The two semilunar (SL) valves are found in the arteries leaving the heart, and they prevent blood flowing 
back from the arteries into the ventricles. 

The "lub-dub" sound of the heartbeat is caused by the closing of the AV valves (lub), and SL valves (dub), 
after blood has passed through them. 



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SL valves 



AV valve 




AV valve 



Figure 2: Blood flows in only one direction in the heart; blood enters the atria, contracting and pushing blood 
into the ventricles, the atria relax, the ventricles fill with blood, contract, and push blood around the body. 

(Sources: http://upload.wikimedia.Org/wikipedia/commons/2/2d/Heart diastole. png, 

http://en.wikipedia.0rg/wiki/lmage:Heart_syst0le.svg, Images by: Wapcaplet in Sodipodi, Wapcaplet, Reytan, 
Mtcv, License: GFDL 1 .2) 

Control of the Heartbeat 

The heart is a made up of cardiac muscle cells. Cardiac cells are able to contract by themselves. They do 
not need help from the nervous system. This is different than skeletal muscle, which needs messages from 
nerve to contract. But the contractions of cardiac muscle still need to be coordinated to make sure the cells 
contract as a group. 

The contraction rate of cardiac muscle is controlled by two small groups of cardiac muscle cells called the 
sinoatrial (SA) and atrioventricular (AV) nodes. The SA node is found in the wall of the right atrium. It starts 
the contraction of muscle cells in the atria. The contracting cells send electrical messages called impulses 
to other muscle cells. The impulses then reach the AV node. The AV node is found in the lower part of the 
right atrium. The AV node conducts the impulses that come from the SA node through the atria to the ven- 
tricles. The impulses then spread around the ventricles and they contract. 

The frequency of the heart's contractions, called the heart rate, can be changed by nervous or hormonal 
signals. Activities such as exercise or getting frightened can make the heart rate increase. After the exercise 
is over, or the fright has passed, the heart rate returns to normal. 

Blood Circulation and Blood Vessels 

There are actually two separate circulation systems within the heart. Both of these together make up the 
complete circulatory system of humans and other animals. Neither system can work alone. These are the 
pulmonary circulation and the systemic circulation. The human heart is made up of two separate pumps, 
the right side which pumps deoxygenated blood into the pulmonary circulation, and the left side which pumps 
oxygenated blood into the systemic circulation. Blood in one circuit has to go through the heart to enter the 
other circuit. 

The blood vessels are an important part of the cardiovascular system. They connect the heart (the pump), 
to every cell in the body. Arteries carry blood away from the heart, while veins return blood to the heart. 
Figure 3 shows the main arteries and veins of the heart. 



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Inferior Vena Cava 



Figure 3: The right side of the heart pumps deoxygenated blood into the pulmonary circulation; the left side 
pumps oxygenated blood into the systemic circulation. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Diagram of the human heart.svg, Image by: Wapcaplet 
in Sodipodi, License: GFDL 1.2) 

The veins that return oxygen- poor blood to the heart are the superior vena cava and the inferior vena cava. 
The pulmonary veins return oxygen-rich blood to the heart. The pulmonary veins are the only veins that 
carry oxygen-rich blood all other veins carry oxygen-poor blood. 

The pulmonary arteries carry oxygen-poor blood away from the heart to the lungs. These are the only arteries 
that carry oxygen-poor blood. The aorta is the largest artery in the body. It carries oxygen-rich blood away 
from the heart. Further away from the heart, the aorta branches into smaller arteries. 

Pulmonary Circulation 

The pulmonary circulation is the part of the cardiovascular system which carries oxygen-poor blood away 
from the heart and brings it to the lungs. Oxygen-poor blood returns to the heart from the body and leaves 
the right ventricle through the pulmonary arteries, which carry the blood to each lung. Once at the lungs, 
the red blood cells release carbon dioxide and pick up oxygen during respiration. The oxygen-rich blood 
then leaves the lungs through the pulmonary veins which return it to the left side of the heart. This completes 
the pulmonary cycle. 

The oxygenated blood is then pumped to the body through the systemic circulation before returning again 
to the pulmonary circulation. 

Systemic Circulation 

The systemic circulation is the part of the cardiovascular system which carries oxygen-rich blood away 
from the heart, to the body, and returns oxygen-poor blood back to the heart. Oxygen-rich blood leaves the 
left ventricle through the aorta, from where it goes to the body's organs and tissues. The blood vessels that 
supply oxygen and nutrients to organs and tissues are much smaller than the vessels that leave the heart. 
Recall that capillaries are the smallest blood vessels. The tissues and organs absorb the oxygen, through 
the capillaries. Oxygen-poor blood is collected from the tissues and organs by tiny veins, which then flow 
into bigger veins. The inferior and superior venae cavae, are the large veins that return oxygen-poor blood 
to the right side of the heart. This completes the systemic cycle. The blood released carbon dioxide and 
gets more oxygen in the pulmonary circulation before returning to the systemic circulation. 



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Coronary Circulation 

Just like every other organ in the body, the heart needs its own blood supply. It gets this blood in the coronary 
circulation. Although blood fills the chambers of the heart, the heart muscle tissue is so thick that it needs 
its own blood vessels to deliver oxygen and nutrients into the muscle. The coronary circulation is part of the 
systemic circulation. The vessels that deliver oxygen-rich blood to the heart muscle are called coronary ar- 
teries. The coronary arteries branch directly from the aorta, just above the heart, as shown in Figure 4. The 
vessels that remove the deoxygenated blood from the heart muscle are known as cardiac veins. Problems 
with the coronary circulation are often referred to as heart disease. 




Aorta 



Coronary arteries 



Figure 4: In coronary circulation, the arteries that bring oxygen to the cardiac cells branch off the aorta; 
heart attacks are caused by blockages of the coronary arteries and blockages in the coronary arteries stop 
oxygen from getting to the heart muscle. 

(Source: http://commons.wikimedia.org/wiki/lmage: HeartJeft_anterior_oblique_diagrams.svg, Image by: 
Patrick J. Lynch and C. Carl Jaffe, M.D., License: CC-BY-SA 2.5) 

The circulation of blood around the body has been studied by people for a long time. The roles of the organs 
of the circulatory system were a mystery for many hundreds of years. For example, it was once believed 
that the left ventricle and arteries were filled with air, and the liver made blood. The pulmonary circulation 
was first discovered by a Syrian physician, Ibn al-Nafis, in 1242. Ibnal-Nafiswas the first person to describe 
the coronary circulation. However, credit for the first description of blood circulation is given to an English 
physician William Harvey. In 1616, Harvey first described the pulmonary and systemic circulation systems 
in detail. 

Lesson Summary 

• The heart is divided into four chambers, the left and right atria and the left and right ventricles. The right 
side of the heart collects deoxygenated blood from the body and pumps it into the lungs where it releases 
carbon dioxide and picks up oxygen. The left-side carries the oxygenated blood back from the lungs, 
into the left side of the heart which then pumps the oxygenated blood throughout the rest of the body. 

• The valves in the heart prevent blood from flowing backward into the heart. The contraction rate of cardiac 
muscle is controlled by two small groups of cardiac muscle cells called the sinoatrial (SA) and atrioven- 
tricular (AV) nodes. 



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• The heart has its own blood supply, which is called the coronary circulation. The heart is fed oxygen-rich 
blood by coronary arteries. Oxygen-poor blood is collected by coronary veins. 

Further Reading I Supplemental Links 

http://en.wikipedia.org/wiki/William_Harvey 
http://thevirtualheart.org/anatomyindex.html 
http://en.wikipedia.org/wiki/Cardiac_cycle 
Review Questions 

1 . Name the four chambers of the heart. (Beginning) 

2. Where does oxygen-poor blood first enter the heart? (Intermediate) 

3. Do ventricles pump blood out of the heart or do they pump blood into the atria? (Beginning) 

4. What is the purpose of the valves in the heart? (Beginning) 

5. What do the AV valves do? (Beginning) 

6. Does the vena cava carry oxygen-poor or oxygen-rich blood? (Beginning) 

7. Why can the heart be considered to be two separate pumps? (Beginning) 

8. How might a hole in the heart wall between the two ventricles affect the circulation of blood? (Challenging) 

9. To what organ or organs does the coronary circulation bring blood? (Beginning) 

10. To what organ or organs does the pulmonary circulation bring blood? (Beginning) 
Vocabulary 

atrioventricular (AV) valves Valves that stop blood from moving from the ventricles back into the atria. 

atrium One of the two small, thin-walled chambers on the top of the heart that blood 

first enters. 

coronary circulation The blood supply that feeds the heart. 

semilunar (SL) valves Found in the arteries leaving the heart; prevents blood flowing back from 

the arteries into the ventricles. 

ventricle One of the two muscular V-shaped chambers that pump blood out of the 

heart. 

Review Answers 

1 . the left and right atria, and the left and right ventricles 

2. oxygen-poor blood first enters the right atrium 

3. ventricles pump blood out of the heart 

4. they prevent blood from flowing backward 

5. they stop blood from moving from the ventricles to the atria 



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6. it carries oxygen-poor blood 

7. the two sides of the heart pump blood into two different circulations, the pulmonary and systemic circulations 

8. oxygen-rich and oxygen-poor blood could mix and the blood going to the body cells would not have as 
much oxygen as it should have 

9. the heart 

10. the lungs 

Points to Consider 

• Identify how the different components of the blood have very different roles in the cardiovascular system. 

• Consider how diet can affect the oxygen-carrying ability of blood. 

Blood 

Lesson Objectives 

• List the components of blood. 

• Identify three functions of blood. 

• Name the oxygen-carrying protein found in red blood cells. 

• Identify the main function of white blood cells. 

• Describe the importance of the ABO blood system. 

• Identify three blood disorders or diseases. 

Check Your Understanding 

• What is the main function of the blood? 

• What is the role of oxygen in aerobic (cellular) respiration? 

Introduction 

Did you know that blood is a tissue? Blood is a fluid connective tissue that is made up of red blood cells, 
white blood cells, platelets, and plasma. It moves around the body through the blood vessels by the pumping 
action of the heart. Oxygen rich blood carried in arteries brings oxygen and nutrient to all the body's cells. 
Oxygen-poor blood carries carbon dioxide and other metabolic wastes away from the cells. As well as the 
transport of gases, nutrients, and wastes, blood has many otherfunctions that are important to homeostasis. 
You will learn more about these functions in this lesson. 

Components of Blood 

Blood is a colloidal solution. A colloidal solution it is made up of particles that are suspended in a fluid. The 
cells in blood are suspended in plasma, the liquid part of blood. The cells that make up the blood are shown 
in Figure 1 . The different components of blood have different roles. Some of the roles of blood include: 

• The defense of the body against infection by microorganisms or parasites. 

• The transport of chemical messages, such as hormones and hormone-like substances. 



462 



The control of body temperature. 
The repair of damage to body tissues. 





ESg 




1 m ^B 


/~\ 




""^^^5 


^^ 3L 



Figure 1: A scanning electron microscope (SEM) image of human blood cells; red blood cells are the flat, 
bowl-shaped cells, the tiny disc-shaped pieces are platelets and white blood cells are the round cells visible 
in the center. 



(Source: http ://visualsonline. cancer.gov/details. cfm?imageid=21 29, 
Harry Schaefer, License: Public Domain) 

Plasma 



Image: Bruce Wetzel, Photographer: 



If you were to filter out all the cells in blood, plasma is what would be left over. Plasma is the golden-yellow 
liquid part of the blood. Plasma is about 90 percent water and about 1 percent dissolved proteins, glucose, 
ions, hormones, and gases. Blood is made up of mostly plasma. The blood cells make up the rest of the 
volume. 

Red Blood Cells 

Red blood cells (RBCs) are flattened disk-shaped cells that carry oxygen. They are the most common 
blood cell in the blood. There are about 4 to 6 million RBCs per cubic millimeter of blood. Each RBC has 
200 million molecules of hemoglobin. Hemoglobin is the protein that carries oxygen. Hemoglobin also gives 
the RBCs their red color. Red blood cells are made in the red marrow of long bones, ribs, skull, and vertebrae. 
Each red blood cell lives for only 120 days (about three months). After this time, they are destroyed in liver 
and spleen. Red blood cells are shown in Figure 2. Mature RBCs do not have a nucleus or other organelles. 



463 




Figure 2: The flattened shape of RBCs helps them to carry more oxygen than if they were rounded. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/1/13/Redbloodcells.jpg, Image by: Drs. Noguchi, 
Rodgers and Schechter of NIDDK, License: Public Domain) 

White Blood Cells 

White blood cells (WBCs) are usually larger than red blood cells. They have a nucleus but do not have 
hemoglobin. White blood cells make up less than one percent of the blood's volume. Most WBCs are made 
in the bone marrow, some mature in the lymphatic system. WBCs defend the body against infection by 
bacteria, viruses and other pathogens. Each WBC type has a specific defense job. Three of the most common 
white blood cells in the body are listed here. 

• Neutrophils can squeeze through capillary walls and swallow particles such as bacteria and parasites. 

• Macrophages can also swallow and destroy old and dying cells, bacteria, or viruses. In Figure 3 a 
macrophage is attacking and swallowing two particles, possibly pathogens. Macrophages also release 
chemical messages that cause the number of WBC to increase. 

• Lymphocytes fight infections by viruses and bacteria. Some lymphocytes attack and kill cancer cells. 
Other lymphocytes attack cells that are infected by viruses. Lymphocytes called B-cells make antibodies. 
Antibodies are chemicals that identify pathogens or other substances as being harmful, or they can 
destroy the pathogen. To learn more about the role of WBCs in protecting the body from infection, go to 
the Diseases and the Body's Defenses chapter. 



464 




Figure 3: A type of WBC, called a macrophage, is attacking and about to swallow two particles. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/1/15/Macrophage.jpg, Image by: Obli, License: 
CC-BY-SA2.0) 

Platelets 

Platelets are very small, but they are very important in blood clotting. Platelets are not cells they are sticky 
little pieces of larger cells. They bud off large cells that stay in the bone marrow. A platelet sits between a 
RBC and a WBC in Figure 4. Platelets carry chemicals that are important for proper blood clotting. When a 
blood vessel gets cut, platelets stick to the injured areas. They release chemicals called clotting factors 
which cause a web of protein fibers to form. This web catches RBCs and forms a clot. This clot stops more 
blood from leaving the body through the cut blood vessel. The clot also stops bacteria from entering the 
body.Platelets survive in the blood for 10 days before they are removed by the liver and spleen. 




Figure 4: A platelet lies between a RBC, at left, and a WBC at right; platelets are little pieces of larger cells, 
called megakaryocytes, which are found in the bone marrow. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/2/24/Red White Blood cells.jpg, License: Public 
Domain) 

Transport of Chemical Messages 

The blood also acts as a messenger delivery service. Chemical messages called hormones are carried and 
delivered by the blood to cells around the body. Hormones are released into the blood by the cells that make 



465 



them and are delivered by the blood to the cells the hormones are made for. An example of a hormone 
transported in the blood is insulin, which regulates the concentration of glucose in the blood. 

Control of Body Temperature 

Your blood system does more than deliver oxygen and nutrients to your body cells. Your blood also moves 
heat (thermal energy) around your body. When your brain senses that your body temperature is increasing, 
it sends messages to the blood vessels in the skin to increase in diameter. Increasing the diameter of the 
blood vessels increases the amount of blood and heat that moves near the skin surface. The heat is then 
released from the skin. 

Blood Clotting 

Blood clotting is a complex process by which blood forms solid clots. As discussed above, clotting is im- 
portant to stop bleeding and begin repair of damaged blood vessels. Blood clotting disorders can lead to an 
increased risk of bleeding or clotting inside a blood vessel. Platelets are important for the proper clotting of 
blood. 

Clotting is started almost immediately when an injury damages the inside lining of a blood vessel. Platelets 
clump together, forming a plug at the site of injury. Then, proteins in the plasma cause a series of chemical 
reactions that form a tough protein called fibrin. The fibrin strands form a web across the platelet plug, 
trapping red blood cells before they can leave through the wound site. This mass of platelets, fibrin, and red 
blood cells forms a clot that hardens into a scab. 

Certain nutrients are needed for the clotting system to work properly. Two of these are calcium and vitamin 
K. Bacteria that live in your intestines make enough vitamin K so you do not need to eat extra vitamin K in 
your food. 

Blood Types 

Blood type is determined by the presence or absence of certain molecules, called antigens, on the surface 
of red blood cells (RBCs). There are four blood types; A, B, AB, and O. 

Type A blood has type A antigens on the RBCs in the blood. 

Type AB blood has A and B antigens on the RBCs. 

Type B has B antigens on the RBCs. 

Type O does not have any antigens (neither A nor B). 

The blood types may also have antibodies for other blood types in their plasma. For example, a person with 
type A blood may have anti-B antibodies (against B antigens), and a person with type O blood can have 
anti-A and anti-B antibodies in their blood. The blood type of a person can be worked out by testing a drop 
of a person's blood using anti-A or anti-B antibodies. 

The ABO blood group system is most important if a person needs a blood transfusion. A blood transfusion 
is the process of putting blood or blood products from one person into the circulatory system of another 
person. 

If a person with type O blood received type A blood, the anti-A antibodies in the person's blood would attack 
the A antigens on the RBCs in the donor blood, as shown in Figure 5. The antibodies would cause the RBCs 
to clump together, and the clumps could block a blood vessel. Such a reaction could be fatal. 



466 



> 4 

Anti -A antibodies 



90 #-©: 



Donor Type A 
blood 



Antibodies stick to antigens, and 
the blood clumps 



Figure 5: A person with type O blood has A and B antibodies in their plasma; if the person was to get type 
A blood instead of type O, Their A antibodies would attach to the antigens on the RBCs and cause them to 
clump together. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/1/1c/Coombs test schematic. png, Image by: A. 
Rad, License: GFDL) 

Blood Donors 

Recall that people with type O blood do not have any antigens on their RBCs. As a result, type O blood can 
be given to people with blood types A, B, or AB. If there are no antigens on the RBCs, there cannot be an 
antibody reaction to the blood. People with type O blood are often called universal donors. 

The blood plasma of AB blood does not contain any anti-A or anti-B antibodies. People with type AB blood 
can receive any ABO blood type. People with type AB positive blood are called universal recipients. Table 
1 lists the antigens and antibodies that define blood type. 

In April 2007 researchers discovered a way to convert blood types A, B, and AB to O. The researchers used 
enzymes to remove the antigens on the surface of the RBCs. This discovery could lead to producing or 
modifying blood cells that can be used as donors to people with all blood types. 



Table 1: Blood Types, Antigens, and Antibodies 



Blood type 


Antigen type 


Plasma antibodies 


Can receive blood from 
types 


Can donate blood to types 


A 


A 


anti-B 


A,0 


A, AB 


B 


B 


anti-A 


B,0 


B, AB 


AB 


A and B 


none 


AB, A, B, 


AB 





none 


anti-A, anti-B 





AB, A, B, 



(Source: Niamh Gray-Wilson) 

Blood Diseases 

Problems can occur with red blood cells, white blood cells, platelets, and other parts of the blood. Many 
blood disorders are genetic, they are inherited from a parent. Some blood diseases are a caused by not 
getting enough of a certain nutrient, while others are cancers of the blood. 

Sickle-Cell Disease 

Sickle cell disease is a blood disease that is caused by abnormally-shaped blood protein hemoglobin. Many 
of the RBCs of a person with sickle cell disease are long and curved (sickle-shaped), as shown in Figure 6. 
The long, sickle-shaped RBCs can have damaged cell membranes, which can cause them to burst. The 



467 



long shape of the cells can cause them to get stuck in narrow blood vessels. This clotting causes oxygen 
starvation in tissues, which causes pain and may cause damage such as stroke or heart attack. People with 
sickle-cell disease are most often well, but can on occasion have painful attacks. The disease is not curable, 
but can be treated with medicines. Heterozygous individuals have an advantage; they are resistant to severe 
malaria. See the Genetics chapter for further discussion. 




Figure 6: The RBCs of a person with sickle cell disease (left) are long and pointed rather than straight like 
normal cells (right); the abnormal cells cannot carry oxygen properly and can get stuck in capillaries. 



(Sources: http://upload.wikimedia.Org/wikipedia/commons/9/92/Sicklecells.jpg, 
dia.org/wiki/lmage:Redbloodcells.jpg, Licenses: Public Domain) 



http://commons.wikime- 



Anemia 

Hemoglobin is the oxygen-carrying molecule found inside RBCs. Anemia results when there is not enough 
hemoglobin in the blood to carry oxygen to body cells. Hemoglobin normally carries oxygen from the lungs 
to the tissues. Anemia leads to a lack of oxygen in organs. Anemia is usually caused by one of three things: 

• A loss of blood volume through a bleeding wound or a slow leak of blood. 

• The destruction of RBCs. 

• Lack of RBC production. 

Anemia may not have any symptoms. Some people with anemia feel weak or tired in general or during ex- 
ercise. They also may have poor concentration. People with more severe anemia often get short of breath 
during activity. Iron-deficiency anemia is the most common type of anemia. It occurs when the dietary intake 
or absorption of iron is less than what is needed by the body. As a result, hemoglobin, which contains iron, 
cannot be made. In the United States, 20 percent of all women of childbearing age have iron deficiency 
anemia, compared with only 2 percent of adult men. The most common cause of iron deficiency anemia in 
young women is blood lost during menstruation. Iron deficiency anemia can be avoided by getting the rec- 
ommended amount of iron in the diet. Anemia is often treated or prevented by taking iron supplements. 

Boys and girls aged between the ages of 9 and 13 should get 9 mg of iron every day. Girls between the 
ages of 14 and 18 should get 15 mg of iron every day. Boys aged between the ages of 14 and 18 should 
get 11 mg of iron every day. Pregnant women need the most iron — 27 mg daily. 

Good sources of iron include shellfish such as clams and oysters. Red meat such as beef is also a good 
source of iron. Non-animal sources of iron include seeds, nuts, and legumes. Breakfast cereals often have 
iron added to them in a process called fortification. Table 1 lists some good sources of iron. Eating vitamin 
C along with the iron-containing food increases the amount of iron that the body can absorb. 



Food 


Milligrams (mg) of Iron 


Canned clams, drained, 3 oz 


23.8 


Fortified dry cereals, about 1 oz 


1.8 to 21.1 



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Roasted pumpkin and squash seeds, 1 oz 


4.2 


Cooked lentils, 14 cup 


3.3 


Cooked fresh spinach, Vi cup 


3.2 


Cooked ground beef, 3 oz 


2.2 


Cooked sirloin beef, 3 oz 


2.0 



(Created by: Niamh Gray-Wilson. Information Source: Centers of Disease Control and Prevention 
http://www.cdc.gOv/nccdphp/dnpa/nutrition/nutrition_for_everyone/iron_deficiency/#lron%20Sources) 

Leukemia 

Leukemia is a cancer of the blood or bone marrow. It is characterized by an abnormal production of blood 
cells, usually white blood cells. Lymphoma is a type of cancer in white blood cells called lymphocytes. There 
are many types of lymphoma. 

Hemophilia 

Hemophilia is the name of a group of sex-linked (X-linked) hereditary diseases that affect the body's ability 
to control blood clotting (see the Genetics chapter). Hemophilia is caused by a lack of clotting factors in the 
blood. Clotting factors are needed for the normal clotting of blood. A person who has hemophilia is initially 
able to make a clot to stop the bleeding, but because fibrin is not produced, the body is unable to keep a 
clot at an injury site. The risk of internal bleeding is also increased in hemophilia, especially into muscles, 
joints, or bleeding into closed spaces. 

Lesson Summary 

• Blood is a colloidal solution that contains red blood cells, white blood cells, and platelets. The cells are 
suspended in plasma. The red blood cells give blood its red color. Blood carries oxygen and nutrients 
to body calls and carries wastes away. It also helps to maintain body temperature and to carry chemical 
messages called hormones around the body. 

• Hemoglobin is the oxygen-carrying protein that is found in red blood cells. White blood cells defend the 
body against infection by bacteria, viruses and other pathogens. Some WBCs swallow pathogens, and 
others produce antibodies that attack and destroy pathogens. 

• Blood type is determined by the presence or absence of certain molecules, called antigens, on the surface 
of red blood cells (RBCs). There are four blood types; A, B, AB, and O. 

• If a person receives the wrong blood type, antibodies in the person's blood would attack the antigens on 
the RBCs in the donor blood. The antibodies would cause the RBCs to clump together, and the clumps 
could block a blood vessel. 

• If a pregnant woman is RhD negative, and the developing baby is RhD positive, the mother can make 
anti-RhD antibodies against the RhD antigens of the fetus. The fetus' red cells are broken down and he 
or she can develop anemia. 

• Sickle cell disease is a blood disease that is caused by abnormally-shaped blood protein hemoglobin. 

• Anemia is a disorder in which there is not enough hemoglobin in the blood to carry oxygen to body cells. 

Review Questions 

1 . What types of cells are found in blood? (Beginning) 

2. What is the liquid part of blood called? (Beginning) 

3. What is the function of platelets? (Beginning) 



469 



4. Identify two functions of blood other than bringing oxygen to body cells. (Beginning) 

5. What is the oxygen-carrying protein found in red blood cells? (Beginning) 

6. Identify two ways that white blood cells defend the body from infection. 

7. (Intermediate) 

8. How are the red blood cells of the different blood groups different? 

9. They have different antigens on the surface of the cells. 

10. Why are people with type O blood called universal donors? (Intermediate) 

11. Why are people with type AB blood called universal recipients? (Intermediate) 

12. Identify three blood disorders or diseases. (Beginning) 

13. How can the shape of the hemoglobin protein in a person with sickle-cell disease affect other body 
systems? (Challenging) 

14. What is a common cause of anemia in young people? (Intermediate) 

15. Identify two good sources of iron in the diet. (Intermediate) 
Further Reading I Supplemental Links 

http://en.wikipedia.org/wiki 
Review Answers 

1 . red blood cells and white blood cells 

2. plasma 

3. platelets help the blood to form a clot 

4. blood help keep the body warm, and it carries hormones around the body 

5. hemoglobin 

6. some white blood cells swallow foreign particles; others make antibodies 

7. red blood cells of type O blood do not have any antigens on them; they will not cause a reaction when 
they are given to a person with a different blood type 

8. type AB blood does not have any antibodies against A and B antigens; people with type AB blood can 
receive any blood type 

9. anemia, leukemia, and hemophilia 

1 0. improperly shaped hemoglobin causes red blood cells to take on a sickle shape; affected red blood cells 
can get stuck in blood vessels and cause clots around the body 

11. lack of iron in the diet 



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12. shellfish and legumes, spinach and red meat 
Vocabulary 



ABO blood type system 

anemia 

antibodies 

blood clotting 
blood transfusion 

fibrin 
hemophilia 

iron-deficiency anemia 

leukemia 

lymphoma 

plasma 

platelets 

red blood cells (RBCs) 

rhesus (Rh) system 



sickle cell disease 
universal donor 

universal recipient 

white blood cells 



Blood group system that is determined by the presence or absence of certain 
molecules, called antigens, on the surface of red blood cells (RBCs); there are 
four blood types in the ABO system: A, B, AB, and O. 

The condition of not having enough hemoglobin in the blood to carry oxygen to 
body cells. 

Proteins that identify pathogens or other substances as being harmful; flow in 
blood; can destroy pathogens by attaching to the cell membrane of the pathogen. 

The complex process by which blood forms solid clots. 

The process of putting blood or blood products from one person into the circula- 
tory system of another person. 

A tough protein that forms strands during the blood clotting process. 

A group of hereditary diseases that affect the body's ability to control blood 
clotting. 

Occurs when the dietary intake or absorption of iron is less than what is needed 
by the body. As a result, hemoglobin, which contains iron, cannot be made. 

Cancer of the blood or bone marrow; characterized by an abnormal production 
of blood cells, usually white blood cells. 

Cancer of white blood cells called lymphocytes. 

The golden-yellow liquid part of the blood. 

Fragments of larger cells that are important in blood clotting. 

Flattened disk-shaped cells that carry oxygen, the most common blood cell in 
the blood. Mature red blood cells do not have a nucleus. 

The second most important blood group system in human blood transfusion. A 
person either has, or does not have the RhD antigen on the surface of their 
RBCs; written as RhD positive (does have the RhD antigen) or RhD negative 
(does not have the antigen). 

A blood disease that is caused by abnormally-shaped blood protein hemoglobin. 

A person with type O positive blood; type O RBC do not have any antigens on 
their membranes and so would not cause an immune reaction in the body of a 
recipient. 

A person with type AB positive blood; the blood plasma of AB blood does not 
contain any anti-A or anti-B antibodies. People with type AB blood can receive 
any ABO blood type. 

Nucleated blood cells that are usually larger than red blood cells; defend the 
body against infection by bacteria, viruses, and other pathogens. 



Points to Consider 

• Why is the blood in veins not under pressure? 

• How can your diet affect the cardiovascular system? 

Health of the Cardiovascular System 

Lesson Objectives 

• Outline the cause of blood pressure in arteries. 



471 



• Identify the healthy range for blood pressure. 

• Describe three types of cardiovascular disease. 

• Identify things you can do to avoid cardiovascular disease. 

Check Your Understanding 

• What is the role of the cardiovascular system? 

Introduction 

The health of your whole body depends on the good health of your cardiovascular system. The health of 
the cardiovascular system (CV system) can be overlooked because damage to the CV system often does 
not have any symptoms. In this lesson you will learn about common health problems with the CV system, 
and how you can work toward having a healthy CV system. 

Blood Vessels and Blood Pressure 

Blood pressure is the force exerted by circulating blood on the walls of blood vessels. The contracting 
ventricles push blood out of the heart under force. The force of the contractions put the blood underpressure. 
The pressure causes the walls of the arteries to move in a rhythmic fashion. Blood in arteries is under the 
greatest amount of pressure. A person's pulse is the throbbing of their arteries that results from the heart 
beat. 

The pressure of the circulating blood gradually decreases as blood moves from the arteries, and into the 
smaller blood vessels. Blood that is in veins is not under pressure. The term blood pressure generally refers 
to the pressure in the larger arteries that take blood away from the heart. Arterial pressure results from the 
force that is applied to blood by the contracting heart, where the blood "presses" against the walls of the 
arteries. 

The systolic arterial pressure is the highest pressure in the arteries. The diastolic arterial pressure is the 
lowest pressure. Arterial pressure is most commonly measured by an instrument called a sphygmomanometer, 
shown in Figure 1 . The height of a column of mercury indicates the pressure of the circulating blood. Many 
modern blood pressure devices no longer use mercury, but values are still reported in millimeters of mercury 
(mmHg). 




Figure 1 : A digital sphygmomanometer is made of an inflatable cuff and a pressure meter to measure blood 
pressure. 



472 



(Source: http://upload.wikimedia.Org/wikipedia/commons/3/3b/BloodPressure.jpg, Image by: Julo, License: 
Public Domain) 

Healthy Blood Pressure Ranges 

In the United States, the healthy ranges for arterial pressure are: 

• Systolic: less than 120 mm Hg 

• Diastolic: less than 80 mm Hg 

Blood pressure is usually written as systolic/diastolic mm Hg. For example, a reading of 120/80 mm Hg, is 
said as "one twenty over eighty". These measures of arterial pressure are not static, they change with each 
heartbeat and during the day. Factors such as age, gender and race also influence blood pressure values. 
Pressure also varies with exercise, emotions, sleep, stress, nutrition, drugs, or disease. 

Studies have shown that people whose systolic pressure is around 1 1 5 mm Hg rather than 1 20 mmHg have 
fewer health problems. Clinical trials have shown that people who have arterial pressures at the low end of 
these ranges have much better long term cardiovascular health. 

Hypertension which is also called high blood pressure, is a condition in which a person's blood pressure 
is always high. Hypertension is said to be present when a person's systolic blood pressure is always 140 
mm Hg or higher, and/or their diastolic blood pressure is always 90 mm Hg or higher. Having hypertension 
increases a person's chance for developing heart disease, having a stroke, and other serious cardiovascular 
diseases. 

Hypertension often does not have any symptoms, so a person may not know they have high blood pressure. 
For this reason hypertension is often called the silent killer. However, hypertension can be easily diagnosed 
and is usually treatable. Treatments for hypertension include diet changes, exercise, and medication. 

Atherosclerosis and Other Cardiovascular Diseases 

A cardiovascular disease (CVD) is any disease that affects the cardiovascular system. But, the term is 
usually used to describe diseases that are linked to atherosclerosis. Atherosclerosis is a chronic inflammation 
of the walls of arteries that causes swelling and a buildup of material called plaque. Plaque is made of cell 
pieces, fatty substances, calcium, and connective tissue that build up around the area of inflammation. As 
a plaque grows it stiffens and narrows the artery, which reduces the flow of blood through the artery, shown 
in Figure 2. 




Figure 2: Athersclorosis is sometimes referred to as hardening of the arteries; plaque build-up reduces the 
blood flow through the artery. 



473 



(Sources: http://commons.wikimedia.0rg/wiki/lmage:Heart_coronary_artery_lesion.jpg,, http://upload.wikime- 
dia.org/wikipedia/commons/f/f1/Atherosclerosis%2C aorta%2C gross pathology PHIL 846 lores.jpg, Image 
by: Patrick J. Lynch and C. Carl Jaffe, M.D., Licenses: CC-BY-2.5, Public Domain) 

A therosclerosis 

Atherosclerosis normally begins in later childhood, and is usually found in most major arteries. It does not 
usually have any early symptoms. Causes of atherosclerosis include a high-fat diet, high cholesterol, 
smoking, obesity, and diabetes. Atherosclerosis becomes a threat to health when the plaque buildup interferes 
with the blood circulation in the heart or the brain. A blockage in the blood vessels of the heart can cause 
a heart attack. Blockage of the circulation in the brain can cause a stroke. According to the American Heart 
Association, atherosclerosis is a leading cause of CVD. 

Coronary Heart Disease 

Cardiac muscle cells are fed by the coronary arteries. Blocked flow in a coronary artery can result in a lack 
of oxygen and the death of heart muscle. Coronary heart disease is the end result of the buildup of plaques 
within the walls of the coronary arteries. Coronary heart disease often does not have any symptoms. A 
symptom of coronary heart disease is chest pain. Occasional chest pain, called angina can happen during 
times of stress or physical activity. The pain of angina means the heart muscle fibers need more oxygen 
than they are getting. 

Most people with coronary heart disease often have no symptoms for many years until they have a heart 
attack. A heart attack happens when the blood supply to a part of the heart is blocked. The cardiac muscle 
that depends on the blood supply from the blocked artery does not get any oxygen. Cardiac muscle fibers 
that is starved of oxygen for more than about five minutes will die. Cardiac muscle does not divide, so dead 
cardiac muscle cells are not replaced. Coronary heart disease is the leading causes of death of adults in 
the United States. Figure 3 shows how a blocked coronary artery can cause a heart attack, and cause part 
of the heart muscle to die. Injured cardiac muscle does not contract as well as healthy tissue, so the heart 
will not work as well as it used to. 




Figure 3: A blockage in a coronary artery stops oxygen getting to part of the heart muscle; areas of the 
heart that depend on the blood flow from the blocked artery are starved of oxygen. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/5/5e/Heart attack diagraming, License: Public 
Domain) 

Stroke 

Atherosclerosis in the arteries of the brain can lead to a stroke. A stroke is a loss of brain function due to 
a blockage of the blood supply to the brain. It can be caused by a blood clot, a free-floating object that gets 
caught in a blood vessel, or by a bleeding blood vessel. 



474 



Risk factors for stroke include advanced age, high blood pressure, having a previous stroke, diabetes, high 
cholesterol, and cigarette smoking. Reducing blood pressure is the most important controllable risk factor 
of stroke. However, many other risk factors, such as avoiding tobacco or quitting tobacco smoking are also 
important. 

Keeping Your Cardiovascular System Healthy 

There are many risk factors that can cause a person to develop CVD. A risk factor is anything that is linked 
to an increased chance of developing a disease or an infection. Some of the risk factors for CVD you cannot 
control, but there are many risk factors you can control. 

Risk factors you cannot control include: 

o Age The older a person is, the greater their chance of developing a cardiovascular disease. 

o Gender Men under age 64 are much more likely to die of coronary heart disease than women, although 
the gender difference declines with age. 

o Genetics Family history of cardiovascular disease increases a person's chance of developing heart 
disease. 

Risk factors you can control include: 

o Tobacco Smoking Giving up smoking or never starting to smoke is the single most effective way of 
reducing the risk of heart disease. 

o Diabetes Having diabetes can cause changes (such as high cholesterol levels) which in themselves 
are risk factors. 

o High Cholesterol Levels High amounts of low density lipids in the blood, also called bad cholesterol, 
are a significant risk factor 

o Obesity Being obese, especially if the fat is deposited mostly in the torso, rather than the hips and 
thighs, increases risk significantly 

o High Blood Pressure Hypertension can cause atherosclerosis 

o Lack of Physical Activity Aerobic activities, such as the one shown in Figure 4, help keep your heart 
healthy To reduce the risk of disease, you should be active for at least 60 minutes a day, five days a week 
(or most days of the week). 

o Poor Eating Habits Eating mostly foods that are nutrient poor (do not have many nutrients other than 
fat or carbohydrate) leads to high cholesterol levels and overweight, among other things. 



475 





Figure 4: Thirty minutes a day of vigorous aerobic activity, such as basketball, is enough to help keep your 
cardiovascular system healthy. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/a/a3/Basket_sky.jpg, Image by: Popperipopp, Li- 
cense: Public Domain) 

Although there are uncontrollable risk factors, a person whose family has a history of CVD does not have 
to develop heart disease. There are many things a person can do to help prevent CVD, even if CVD is in 
their family. A person who is physically active every day, eats healthfully, and avoids tobacco can lower their 
chances of developing a CVD. 

Men have a higher rate of cardiovascular disease than women do, but it is the number one health problem 
for women in industrialized countries. The risk for older women (in late adulthood) is almost equal that of 
older men. 

Cardiovascular Disease Awareness: What You Can Do 

Being active every day and eating healthfully are two of the most important things you can do to maintain 
a healthy cardiovascular system. Avoiding tobacco is also very important. You do not to be on a sports team 
or join a gym to be physically active. For example, shooting hoops at your school or local basketball courts 
can help keep your heart healthy. Aerobic activities are activities that cause your heart to beat faster and 
allow your muscles to use oxygen to get energy to contract. When done regularly, aerobic activities increase 
the size of the heart so it pumps blood around the body more efficiently. Aerobic activities also help to keep 
blood vessels healthy. To stay healthy, teens and children should be active for at least 60 minutes most 
days of the week. 

Limiting the amount of saturated fat in your diet can also keep your heart healthy. Saturated fats are found 
in dairy foods, meats, cookies, pies, some chocolates, and ice cream. Saturated fats are usually solid at 
room temperature. Fat gives food flavor and texture. Saturated fats occur naturally in foods that come from 
animals, such as meat and milk, but they are often added to baked products such as cookies, shown in 
Figure 5, to give the foods flavor and texture. Not all fats are harmful to the cardiovascular system. Fats 
called monounsaturated and polyunsaturated fats are needed by the body, and should make up most of the 
fats that you eat in your diet. Monounsaturated and polyunsaturated fats are found in plants and fish, and 
are usually liquid at room temperature. To learn more about the importance of fats in your diet, read the 
Choosing Healthful Foods lesson of the Food and the Digestive System chapter. 



476 




Figure 5: The USDA's MyPyramid recommends that you limit the amount of such foods in your diet to oc- 
casional treats; some foods containing saturated fats may contain other nutrients. 

(Source: http://upload.wikimedia.Org/wikipedia/commons/2/24/Christmas Cookies.jpg, Image by: Keksbaggem, 
License: CC-BY-SA 2.0) 

Cardiovascular diseases are called lifestyle diseases because they are caused mostly by everyday choices 
that people make, such as what to eat for dinner, or what to do during their free time. For example, watching 
TV with your dog does not involve much moving around so it does not exercise the body, whereas bringing 
the dog for a walk outside exercises both of you. Decisions that you make today and everyday - those of 
developing healthy lifelong habits - will affect your cardiovascular health many years from now. 

Many studies have shown that plaque build-up starts in the teen years. However, teens are more concerned 
about risks such as HIV, accidents, and cancer than cardiovascular disease. One in three people will die 
from complications due to atherosclerosis. For this reason there is an emphasis on the prevention of CVD 
through risk reduction. For example, healthy eating, regular physical activity, and avoidance of smoking can 
greatly decrease a person's chance of developing a CVD. 

Lesson Summary 

• Blood pressure is the force exerted by circulating blood on the walls of blood vessels. The force of the 
contractions put the blood under pressure. Blood pressure is measured by an instrument called a 
sphygmomanometer. 

• In the United States the healthy ranges for systolic pressure is less than 120 mm Hg and a diastolic 
pressure of less than 80 mm Hg. Hypertension is a condition in which a person's blood pressure is always 
high. 

• A cardiovascular disease (CVD) is any disease that affects the cardiovascular system. Atherosclerosis, 
coronary heart disease, and stroke are examples of CVDs. 

• Cardiovascular diseases are lifestyle diseases, they are mostly caused by lifestyle choices that people 
make. Having a poor diet and not getting enough exercise are two major causes of CVD. 

Further Reading I Supplemental Links 

http://www.presidentschallenge.org/; http://mypyramid.gov 

http://www.cdc.gov/youthcampaign/marketing/tweens/yellowball/index.htm 

http://www.cdc.gov/nccdphp/dnpa/physical/everyone/recommendations/index.htm 



477 



http://en.wikipedia.org/wiki/Aerobic_exercise; http://www.cdc.gov/bloodpressure 
Review Questions 

1 . What is the cause of blood pressure? (Beginning) 

2. How is the pulse related to blood pressure? (Intermediate) 

3. Is the blood in veins under pressure? Explain your answer. (Intermediate) 

4. What is the healthy range for blood pressure? (Beginning) 

5. When is a person considered to have hypertension? (Beginning) 

6. Why is hypertension called a silent killer? (Beginning) 

7. A stroke is often called a brain attack, in a similar way to a heart attack. How are these two things similar? 
(Intermediate) 

8. What is atherosclerosis? (Beginning) 

9. What is a risk factor? (Beginning) 

10. What is the difference between a controllable risk factor and an uncontrollable risk factor? (Intermediate) 

11 . Why are cardiovascular diseases called lifestyle diseases? (Intermediate) 

12. Identify three things a person could do to reduce their chances of developing a CVD. (Intermediate) 
Vocabulary 

angina Chest pain caused by the lack of oxygen to the heart muscle; can happen 

during times of stress or physical activity. 

atherosclerosis A chronic inflammation of the walls of arteries that causes swelling and a 

buildup of material called plaque. 

blood pressure The force exerted by circulating blood on the walls of blood vessels. 

cardiovascular disease Any disease that affects the cardiovascular system, although the term is 
(CVD) usually used to describe diseases that are linked to atherosclerosis. 

coronary heart disease The end result of the buildup of plaques within the walls of the coronary arter- 
ies. 

heart attack Event that occurs when the blood supply to a part of the heart is blocked. 

hypertension Also called high blood pressure; a condition in which a person's blood pressure 

is always high; the systolic blood pressure is always 140 mm Hg or higher, 
and/or their diastolic blood pressure is always 90 mm Hg or higher. 

plaque Cell pieces made up of fatty substances, calcium, and connective tissue that 

build up around the area of inflammation; builds up on the lining of blood 
vessels. 

risk factor Anything that is linked to an increased chance of developing a disease or an 

infection. 

stroke A loss of brain function due to a blockage of the blood supply to the brain. 

Review Answers 

1 . contracting ventricles push the blood out of the heart under pressure; the pressure from the contractions 
causes the blood to exert a force on the artery walls 



478 



2. the pulse is the rhythmic movement of the artery walls which are caused by the contracting ventricles 

3. no, the blood in veins is not under pressure; only the blood leaving the heart is under pressure, and veins 
bring blood back to the heart 

4. less than 120/80 mmHg 

5. when the person has blood pressure over 120/80 mmHg 

6. hypertension is the cause of many cardiovascular diseases, but it does not have any symptoms 

7. they are both the result of a stoppage of the blood supply to the organs (brain and heart) 

8. atherosclerosis is a chronic inflammation of the walls of arteries that causes swelling and a buildup of 
material called plaque 

9. a risk factor is anything that is linked to an increased chance of developing a disease or an infection 

1 0. a controllable risk factor is a risk factor that a person can change; an uncontrollable risk factor is a risk 
factor that a person cannot change 

11 . most CVDs are caused by decisions people make in their everyday life, such as what they eat and their 
levels of activity 

12. avoid smoking, eat less saturated fat, and be active for 60 minutes a day, most days of the week 

Points to Consider 

• Do you think there is a relationship between the cardiovascular system and the respiratory system? What 
could it be? 

• Do you think hypertension affects the ability of the blood to release carbon dioxide and pick up oxygen 
in the lungs? Why? 



479 



480 



19. Respiratory and Excretory Systems 



Respiratory System 

Lesson Objectives 

• Identify the parts of the respiratory system. 

• Identify the main function of the respiratory system. 

• Describe how breathing works. 

• Outline how the respiratory system and the cardiovascular system work together. 

• Identify how breathing and cellular respiration are connected. 

Check Your Understanding 

• What is an organ system? 

• What is the role of the circulatory system? 

• How does your blood get oxygen? 

Introduction 

You breathe mostly without thinking about it. But, do you remember how uncomfortable you felt the last time 
you had a cold or a cough? You usually do not think about your respiratory system or how it works until 
there is a problem with it. Every cell in your body depends on your respiratory system. In this lesson, you 
will learn how your respiratory system works with your cardiovascular system to bring oxygen to every cell 
in your body. 

Roles of the Respiratory System 

Your respiratory system is made up of the tissues and organs that allow oxygen to enter and carbon dioxide 
to leave your body. These structures include your nose, mouth, larynx, pharynx, lungs, and diaphragm. 
These structures are shown in Figure 1 . The main function of the respiratory system is to bring oxygen into 
the body and releases carbon dioxide into the atmosphere. 



481 



Pharynx 






^~~~~^ 1 /j=l^y Nose 


Larynx 




f 


-A Irl ^"{~ Mouth 


Lungs 


/~~~Pff CL^ Trachea 




x % 


i 




l ' /L 


$Lm\ 




Diaphragm \j 







Figure 1: The respiratory system; air moves in through the nose and mouth, and down the trachea which 
is a long straight tube in the chest. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Respirat0ry system. svg, Image by; Theresa Knott, License: 
CC-BY-SA2.5, GFDL 1.2) 



Parts of the Respiratory System 

Figure 1 shows many of the structures of the respiratory system. Each of the parts has a specific job. 
parts of the respiratory system include: 



The 



The diaphragm is a sheet of muscle that extends across the bottom of the rib cage. It performs an important 
function in respiration. When the diaphragm contracts the chest volume gets larger and the lungs take in 
air. When the diaphragm relaxes, the chest volume gets smaller and air is pushed out of the lungs. 

The nose and nasal cavity filters, warms, and moistens the inhaled air. The nose hairs and mucus produced 
by the cells that line the nose catch airborne particles and prevent them from reaching the lungs. 

Behind the nasal cavity, air next passes through the pharynx, a long tube that is shared with the digestive 
system. Both food and air pass through the pharynx. A flap of connective tissue called the epiglottis closes 
over the trachea when food is swallowed to prevent choking or inhaling food. 

The larynx, also called the voicebox, is found just below the point at which the pharynx splits into the trachea 
and the esophagus. Your voice comes from your larynx. Air from the lungs passes across thin membranes 
in the larynx and produces sound. 

The trachea, or wind pipe, is a long tube that leads down to the chest where it divides into the right and left 
bronchi in the lungs. The bronchi branch out into smaller bronchioles in each lung. 

The bronchioles lead to the alveoli. Alveoli are the little sacs at the end of the bronchioles. They look like 
little bunches of grapes at the end of the bronchioles, as shown in Figure 2. Most of the gas exchange occurs 
in the alveoli. Gas exchange is the movement of oxygen across a membrane and into the blood and the 
movement of carbon dioxide out of the blood. 



482 



Bronchi, Bronchial Tree, and Lungs 



Larynx 



Pulmonary vein 



Pulmonary artery --*\\ 
— Trachea 





Alveolar duct Alveo 



Cardiac note!" 



Figure 2: The alveoli are the tiny grape-like structures in the lungs and the sites of gas exchange. 

(Source: http://training.seer.cancer.gov/module anatomy/images/illu bronchi lungs.jpg) 

How We Breathe 

Most of the time, you breathe without thinking of it. Breathing is mostly an involuntary action that is controlled 
by a part of your brain that also controls your heart beat. If you swim, do yoga, or sing, you know you can 
also control your breathing. 




Figure 3: Being able to control breathing is important for many activities, such as swimming. The man in 
the photograph is exhaling before he surfaces the water. 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/6/6b/Air bubbles in a pool as a man surfaces for 
air.jpg, Image by: David Shankbone, License: CC-BY-SA 3.0, GFDL 1.2) 

Taking air into the body through the nose and mouth is called inhalation. Pushing air out of the body through 
the nose or mouth is called exhalation. The man in Figure 3 is exhaling before he surfaces in the pool water. 
The lungs cannot move by themselves. As mentioned above, air moves into and out of the lungs by the 
movement of muscles. The diaphragm and rib muscles contract and relax to move air in to and out of the 
lungs. 

During inhalation, the diaphragm contracts and moves downward. The rib muscles contract and cause the 
ribs to move outward. This causes the chest volume to increase. Because the chest volume is larger, the 
air pressure inside the lungs is less than the air pressure outside. This difference in air pressures causes 
air to be sucked into the lungs. When the diaphragm and rib muscles relax, air is pushed out of the lungs. 



483 



Exhalation is normally a passive process, similar to letting the air out of a balloon. 

The walls of the alveoli are very thin and are permeable to gases. The alveoli are lined with capillaries, the 
walls of which are also thin enough to allow gas exchange. These capillaries are shown in Figure 4. Oxygen 
diffuses from the alveoli to the blood in the capillaries that surround the alveoli. At the same time, carbon 
dioxide diffuses in the opposite direction, from capillary blood to the alveoli. At this point, the pulmonary 
blood is oxygen-rich, and the lungs are holding carbon dioxide. Exhalation follows, thereby ridding the body 
of the carbon dioxide and completing the cycle of respiration. 




Figure 4: The bronchi and alveoli; during respiration, oxygen gets pulled into the lungs and enters the blood 
by passing across the thin alveoli membranes and into the capillaries. 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/O/Of/Bronchial anatomy.jpg, Image by: Patrick J. 
Lynch and C. Carl Jaffe, M.D., License: CC-BY-SA 2.0) 

Breathing and Respiration 

When you breath in, oxygen is drawn in through the mouth and down into the lungs. The oxygen then passes 
across the thin lining of the capillaries and into the blood. The oxygen molecules are carried to the body 
cells by the blood. Carbon dioxide from the body cells is carried by the blood to the lungs where it is released 
into the air. The process of getting oxygen into the body and releasing carbon dioxide is called respiration. 

Sometimes breathing is called respiration. But, there is much more to respiration than just breathing. There 
are actually two parts to respiration. The movement of oxygen into the body and carbon dioxide out of the 
body is called external respiration. The exchange of gases between the blood and the cells of the body is 
celled internal respiration. 

The Journey of a Breath of Air 

Breathing is only part of the process of delivering oxygen to where it is needed in the body. Gas exchange 
occurs in the alveoli by passive diffusion of gases between the alveoli and the blood in the capillaries of the 
lungs. The passive diffusion of oxygen and carbon dioxide is shown in Figure 5. 

Recall that diffusion is the movement of substances from an area of higher concentration to an area of lower 
concentration. The difference between the high concentration of oxygen (0 2 ) in the alveoli and the low 2 

concentration of the blood in the capillaries is enough to cause 2 molecules to diffuse across the thin walls 

of the alveoli and capillaries and into the blood. Carbon dioxide (C0 2 ) moves out of the blood and into the 

alveoli in a similar way. 



484 



After leaving the lungs, the oxygenated blood returns to the heart to be pumped through the aorta and around 
the body. The oxygenated blood travels through the aorta, to the smaller arteries and finally to the capillaries 
where gas exchange occurs. The oxygen molecules move out of the capillaries and into the body cells. 
While oxygen diffuses from the capillaries and into body cells, carbon dioxide diffuses from the cells into the 
capillaries. 



I Blood In 




Blood Out 

Figure 5: Gas exchange is the movement of oxygen into the blood and carbon dioxide out of the blood. 

(Source: http://commons.wikimedia.org/wiki/lmage: Alveoli. svg, Image by: Mr TS88 Duel, License: GFDL, 
CC-BY-2.5) 

Breathing and Cellular Respiration 

The oxygen that arrives at the cells from the lungs is used by the cells to release the energy stored in 
molecules of sugar. Cellular respiration is the process of breaking down glucose to release energy (see 
the Cell Functions chapter). The waste products of cellular respiration include carbon dioxide and water. 
The carbon dioxide molecules move out of the cells and into the capillaries that surround the cells. The 
carbon dioxide is removed from the body by the lungs. 

Lesson Summary 

• Your respiratory system is made up of the tissues and organs that allow oxygen to enter and carbon 
dioxide to leave your body. These structures include your nose, mouth, larynx, pharynx, lungs, and di- 
aphragm. The main function of the respiratory system is to bring oxygen into the body and releases 
carbon dioxide into the atmosphere. During inhalation, the diaphragm contracts and moved downward. 
The rib muscles contract and cause the ribs to move outward, causing the chest volume to increase. Air 
pressure inside the lungs is less than the air pressure outside so air is sucked into the lungs. When the 
diaphragm and rib muscles relax, air is pushed out of the lungs. Exhalation is normally a passive process. 

• Oxygen enters the lungs, passes through the alveoli and into the blood. The oxygen is carried around 
the body in blood vessels. In a similar way, carbon dioxide, a waste product, moves into the blood capil- 
laries by passive diffusion and is brought to the lungs in the pulmonary circulation. The carbon dioxide 
is released into the air during exhalation. The oxygen that arrives from the lungs is used by the cells 
during cellular respiration to release the energy stored in molecules of sugar. A waste product of cellular 
respiration, carbon dioxide, is removed from the body by exhalation. 

Review Questions 

1. Name the parts of the respiratory system. (Beginning) 

2. What is the main function of the respiratory system? (Beginning) 

3. A classmate says that the lung muscles cause the lungs to move during breathing. Do you agree with 
your classmate? (Intermediate) 



485 



4. How do the respiratory system and the cardiovascular system work together? (Intermediate) 

5. Breathing is an involuntary action. Does this mean that you cannot control your breathing? (Challenging) 

6. In what part of the lung does gas exchange occur? (Beginning) 

7. What is the difference between breathing and respiration? (Intermediate) 

8. Identify how breathing and cellular respiration are connected. (Intermediate) 

9. What is the important gas that is carried into the lungs during inhalation? (Beginning) 

10. What is the name of the waste gas that is released during exhalation? (Beginning) 

11 . If a disease caused the alveoli to collapse, how might this affect a person's health? (Challenging) 
Further Reading I Supplemental Links 

http://en.wikipedia.org/wiki 
Vocabulary 

alveoli Little "sacs" at the end of the bronchioles where most of the gas exchange occurs. 

diaphragm A sheet of muscle that extends across the bottom of the rib cage. When the diaphragm 

contracts the chest volume gets larger and the lungs take in air; when the diaphragm 
relaxes, the chest volume gets smaller and air is pushed out of the lungs. 

epiglottis A flap of connective tissue that closes over the trachea when food is swallowed to 

prevent choking or inhaling food. 

exhalation Pushing air out of the body through the nose or mouth. 

external respiration The movement of oxygen into the body and carbon dioxide out of the body. 

gas exchange The movement of oxygen across a membrane and into the blood and the movement 

of carbon dioxide out of the blood. 

inhalation Taking air into the body through the nose and mouth. 

internal respiration The exchange of gases between the blood and the cells of the body. 

larynx Found just below the point at which the pharynx splits into the trachea and the 

esophagus. Your voice comes from your larynx; air from the lungs passes across 
thin membranes in the larynx and produces sound; also called the voicebox. 

pharynx A long tube that is shared with the digestive system; both food and air pass through 

the pharynx. 

respiration The process of getting oxygen into the body and releasing carbon dioxide. 

trachea A long tube that leads down to the chest where it divides into the right and left bronchi 

in the lungs; also called the windpipe. 

Review Answers 

1. nose, mouth, larynx, pharynx, lungs, diaphragm, bronchus, bronchiole, alveolus 

2. to control the direction of airflow during inhalation and exhalation 

3. no, I do not agree, the lungs do not have muscles; it is the movement of the diaphragm and the rib muscles 
that cause the lungs to move during breathing 

4. the respiratory system brings oxygen into the lungs from outside, the oxygen enters the blood from the 
alveoli and the blood brings the oxygen to the body cells, and the blood returns carbon dioxide to the lungs 
so that the lungs can release it into the air 



486 



5. breathing is an automatic process; a person does not have to think about it however, a person can also 
control their breathing and hold their breath, or control the rate of breathing 

6. across the thin walls of the alveoli 

7. breathing involves moving air into and out of the lungs; respiration involves the movement of oxygen from 
the lungs to the body cells and the removal of carbon dioxide from the cells and to the lungs 

8. cellular respiration requires oxygen, and breathing allows the oxygen to get to the alveoli so it can be 
transported to each cell in the body 

9. oxygen 

10. carbon dioxide 

11. gas exchange would not take place across the walls of the alveoli; enough oxygen would not be able to 
enter the blood, and the person would be out of breath 

Points to Consider 

• How do you think the health of your respiratory system might affect the health of other body systems? 



Health of the Respiratory System 

Lesson Objectives 

• Identify the organs affected by a respiratory disease. 

• Identify how a respiratory disease can affect the rest of the body. 

• Describe how asthma affects breathing. 

• Outline how smoking affects the respiratory system. 

• Identify what you can do to keep your respiratory system healthy. 

Check Your Understanding 

• What is the role of the respiratory system? 

• What are some of the organs of the respiratory system? 

Introduction 

Most of the time your respiratory system works well, and you don't notice it doing its job. But your respiratory 
system can sometimes be knocked out of homeostasis. Recall that homeostasis is the balancing act your 
body performs that keeps conditions in your body stable. Anything that disrupts the respiratory system from 
doing its job disrupts homeostasis. When homeostasis no longer exists, there is disease. There are many 
causes of respiratory diseases, and many ways to treat such diseases. In this lesson you will learn about 
some of the most common respiratory diseases, and what you can do to help avoid them. You will also learn 
how the use of tobacco disrupts homeostasis, which leads to some of the most serious respiratory diseases. 

Respiratory System Disease 

In general, diseases that last a short time are called acute diseases. Other diseases can last for a long time, 
perhaps years. Diseases that last for a long time are called chronic diseases. Both acute and chronic diseases 
affect the respiratory system. Respiratory diseases are diseases of the lungs, bronchial tubes, trachea, 



487 



nose, and throat. These diseases can range from a mild cold to a severe case of bacterial pneumonia. 
Respiratory diseases are common and may cause illness or death. Some respiratory diseases are caused 
by bacteria while others are caused by viruses, environmental pollutants such as tobacco smoke, or are 
hereditary. 




Figure 1: This boy is suffering from whooping cough (also known as pertussis) which gets its name from 
the loud whooping sound that is made when the person inhales during a coughing fit. 

(Source: http://en.wikipedia.Org/wiki/lmage:Pertussis.jpg, License: Public Domain) 

Bronchitis 

Bronchitis is an inflammation of the bronchi. Acute bronchitis is usually caused by viruses or bacteria and 
may last several days or weeks. It is characterized by a cough that produces phlegm (mucus). Symptoms 
include shortness of breath and wheezing, which are related to the inflammation of the airways. Acute 
bronchitis is usually treated with antibiotics. 

Chronic bronchitis may not be caused by a bacterium or a virus. Chronic bronchitis is defined as having a 
cough that produces phlegm, for at least three months in a two-year period. Tobacco smoking is the most 
common cause of chronic bronchitis, but it can be caused by environmental pollution such as smog and 
dust. It is generally part of a syndrome called chronic obstructive pulmonary disease (COPD), which we will 
learn about later. Treatments for bronchitis include antibiotics and steroid drugs to reduce inflammation. 

Asthma 

Asthma is a chronic illness in which the bronchioles are inflamed and become narrow, as shown in Figure 
2. The muscles around the bronchioles contract which narrows the airways further. Large amounts of mucus 
are also made by the cells that line the lungs. A person with asthma has difficulty breathing. Their chest 
feels tight and they wheeze. 

Asthma can be caused by different things such as exposure to an allergen. An allergen is any antigen that 
is not an infectious organism. Allergens can cause allergic reactions. Common allergens that cause asthma 
are mold, dust, or pet hair. Asthma can also be caused by cold air, warm air, moist air, exercise, or stress. 
The most common asthma triggers are viral illnesses such as the common cold. The symptoms of asthma 



488 



can usually be controlled with medicine. Bronchodilators are drugs that reduce inflammation of the bronchioles 
allowing air through. 

Asthma is not contagious and cannot be passed onto other people. Sometimes people with asthma are 
afraid that being active could cause them to have an asthma attack. Having asthma does not mean that you 
have to miss out on being active. Many teens that have asthma are active every day. Asthma cannot be 
cured, but is treatable with medicines. Children and adolescents who have asthma can still lead active lives 
if they control their asthma. Asthma can be controlled by taking medication and by avoiding contact with 
environmental triggers for asthma. 

Before an Asthma Episode After an Asthma Episode 

Muscles around 
the airway 
Airway /-" (£-=3, Z>C with mucus /^^sc^/^v™"*™ 171 





Airway? swel 



Figure 2: The two reactions that lead to asthma are when the bronchioles swell and the muscles around 
the bronchioles contract. 

(Source: http://upl0ad.wikimedia.0rg/wikipedia/c0mm0ns/e/el/Asthma before-after.png, License: Public 
Domain) 

Pneumonia 

Pneumonia is an illness in which the alveoli become inflamed and flooded with fluid. Pneumonia is a restrictive 
respiratory disease. Gas exchange cannot happen properly across the alveoli membranes. Pneumonia can 
be caused by many things. Infection by bacteria, viruses, fungi, or parasites can cause pneumonia. An injury 
caused by chemicals or a physical injury to the lungs can also cause pneumonia. Symptoms of pneumonia 
include cough, chest pain, fever, and difficulty in breathing. Treatment depends on the cause of pneumonia. 
Bacterial pneumonia is treated with antibiotics. 

Pneumonia is a common illness which occurs in all age groups, and is a leading cause of death among the 
elderly and people who are chronically and terminally ill. Vaccines to prevent certain types of pneumonia 
are available. 

Tuberculosis 

Tuberculosis (TB) is a common and often deadly infectious disease caused by a type of bacterium called 
mycobacterium. Tuberculosis most commonly attacks the lungs but can also affect other parts of the body. 
Mycobacteria in the alveoli cause an immune reaction in the body that damages the alveoli. TB is a chronic 
disease, but most people who become infected do not develop the full disease. The TB mycobacteria are 
spread in the air when people who have the disease cough, sneeze or spit. To help prevent the spread of 
the disease, public health notices, such as the one in Figure 3, reminded people how to stop the spread of 
the disease. Currently, drug resistant forms of TB are creating a new challenge for health professionals. 



489 



PREVENT DISEASE 







t 



CARELESS 

SpnriMi, Cokjhimj, Sneezing, 

SPREAD INFLUENZA 
and TUBERCULOSIS 



f 



Figure 3: A public health notice from the early 20th century reminded people that TB could be spread very 
easily. 

(Source: http://en.wikipedia.Org/wiki/lmage:TB poster.jpg, License: Public Domain) 

Cancer 

Lung cancer is a disease where the cells that line the lungs grow out of control. The growing mass of cells 
pushes into nearby tissues and can affect how these tissues work. Lung cancer, which is the most common 
cause of cancer-related death in men and the second most common in women, is responsible for 1 .3 million 
deaths worldwide every year The most common symptoms are shortness of breath, coughing (including 
coughing up blood), and weight loss. The most common cause of lung cancer is exposure to tobacco smoke. 

Emphysema 

Emphysema is a chronic lung disease caused by loss of elasticity of the lung tissue. The surfaces of healthy 
alveoli are springy and elastic. They stretch out a little when full of air and relax when air leaves them. But 
the breakdown of the tissues that support the alveoli and the capillaries that feed the alveoli cause the 
alveoli to become hard and stiff. Eventually the walls of the alveoli break down and the alveoli become larger. 
When alveoli become larger, the amount of oxygen that can enter the blood with each breath is reduced. 
Much of the oxygen that gets into the large alveoli cannot be absorbed across the alveoli walls into the blood. 
Symptoms of emphysema include shortness of breath on exertion (usually when climbing stairs or a hill). 
Damage to the alveoli, which can be seen in Figure 4, is not curable. Smoking is a leading cause of emphy- 
sema. 



490 





Figure 4: The lung of a smoker who had emphysema (left); the black areas are enlarged alveoli and tar, a 
sticky, black substance found in tobacco smoke is evident, and (right) COPD (Chronic obstructive pulmonary 
disease), a tobacco-related disease that is characterized by emphysema. 

(Sources: http://upload.wikimedia.0rg/wikipedia/commons/a/ac/Centrilobular emphysema 865 lores.jpg, 
http://upload.wikimedia.Org/wikipedia/en/5/59/Copd versus healthy lung.jpg, Licenses: Public Domain, COPD) 

Causes of Respiratory Diseases 

Pathogens 

Many respiratory diseases are caused by pathogens. A pathogen is an organism that causes disease in 
another organism. Certain bacteria, viruses, and fungi are pathogens of the respiratory system. The common 
cold and flu are caused by viruses. The influenza virus that causes the flu is shown in Figure 5. Tuberculosis, 
whooping cough, and acute bronchitis are caused by bacteria. The pathogens that cause colds, flu, and TB 
can be passed from person to person by coughing, sneezing, and by spitting. 




Figure 5: This is the influenza virus that causes the flu; the CDC (The Center for Disease Control and Pre- 
vention) recommends that children between the ages of 6 months and 19 years get a flu vaccination each 
year. 

(Source: http://commons.wikimedia.0rg/wiki/lmage:lnfluenza virus particle color.jpg, License: Public Domain) 

Pollution 

Air quality is related to several respiratory diseases. Asthma, heart and lung diseases, allergies, and several 
types of cancers are all linked to air quality. Air pollution can be caused by outdoor pollution or indoor pollution. 
Outdoor air pollution can be caused by car exhaust fumes, smoke from factories and forest fires, volcanoes, 
and animal feces. Some of the pollutants of concern include particulates, carbon dioxide, sulfur oxides, and 



491 



lead. These pollutants contain tiny particles that can get "stuck" in the lining of the respiratory system and 
irritate the lungs. Indoor air pollution can be caused by tobacco smoke, dust, mold, insects, rodents, and 
cleaning chemicals. 

Lifestyle Choices 

Smoking is the major cause of chronic respiratory disease as well as cardiovascular disease and cancer. 
Exposure to tobacco smoke, by smoking or by breathing air that contains tobacco smoke is the leading 
cause of preventable death in the U.S. Regular smokers die about 10 years earlier than nonsmokers do. 
The Centers for Disease Control and Prevention (CDC) describes tobacco use as "the single most important 
preventable risk to human health in developed countries and an important cause of premature death 
worldwide. 

Dangers of Smoking 

Tobacco use, particularly cigarette smoking, is the single most preventable cause of death in the United 
States. Cigarette smoking alone is directly responsible for approximately 30 percent of all cancer deaths 
annually in the United States. The main health risks of using tobacco are linked to diseases of the cardio- 
vascular system and respiratory system. Cardiovascular diseases caused by smoking include heart disease 
and stroke. Diseases of the respiratory system that are caused by exposure to tobacco smoke include em- 
physema, lung cancer, and cancers of the larynx and mouth. Cigarette smoking causes 87 percent of lung 
cancer deaths. Smoking and using tobacco is also linked to the risk of developing other types of cancer 
such as pancreatic and stomach cancer. 

Cigarettes, like the ones shown in Figure 6, are a major source of indoor air pollution. Cigarette smoke 
contains about 4,000 substances, including over 60 cancer-causing chemicals. Many of these substances, 
such as carbon monoxide, tar, arsenic, and lead, are toxic to the body. Non-smokers can also be affected 
by tobacco smoke. Exposure to secondhand smoke, also known as environmental tobacco smoke (ETS), 
greatly increases the risk of lung cancer and heart disease in nonsmokers. 

Chronic obstructive pulmonary disease (COPD) is a disease of the lungs in which the airways become 
narrowed. This leads to a limitation of the flow of air to and from the lungs causing shortness of breath. The 
limitation of airflow usually gets worse over time. COPD is most commonly caused by smoking. Gases and 
particles in tobacco smoke trigger an abnormal inflammatory response in the lung. The inflammatory response 
in the larger airways is known as chronic bronchitis. In the alveoli, the inflammatory response causes the 
breakdown of the tissues in the lungs, leading to emphysema. 




Figure 6: Tobacco use, particularly cigarette smoking, is the single most preventable cause of death in the 
United States. 



492 



(Source: http://upload.wikimedia.Org/wikipedia/commons/7/7a/Spitkid.jpg, Image by: spitkid, License: Public 
Domain) 

Keeping Your Respiratory System Healthy 

Many of the diseases related to smoking are called lifestyle diseases, diseases that are caused by choices 
that people make in their daily lives. For example, the choice to smoke can lead to cancer in later life. But, 
there are many things you can do to help keep your respiratory system healthy. Some of these are listed 
here: 

Avoid Smoking 

Never smoking or quitting now are the most effective ways to reduce your risk of developing chronic respi- 
ratory diseases such as cancer. 

Eat Well, Exercise Regularly, and Get Rest 

Eating a healthful diet, getting enough sleep, and being active every day can help keep your immune system 
strong. 

Wash Your Hands 

Washing your hands often, and after sneezing, coughing or blowing your nose help to protect you and others 
from diseases. Washing your hands for 20 seconds with soap and warm water can help prevent colds and 
flu. Some viruses and bacteria can live from 20 minutes up to 2 hours or more on surfaces like cafeteria 
tables, doorknobs, and desks. Figure 7 is a public health notice that shows people how to prevent the spread 
of respiratory diseases. 

Avoid Contact with Others When Sick 

Do not go to school or to other public places when you are sick. You risk spreading your illness to other 
people and getting sicker if you catch something else. 

Visit Your Doctor 

Getting the recommended vaccinations can help prevent diseases such as whooping cough and flu. Seeking 
medical help for diseases such as asthma can help control the severity of the disease. 



493 



Stop the spread of germs that make you and otheis sick! 



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Figure 7: Cover yot/r Cough; Clean your Hands is a public health campaign that reminds people of the 
quickest and easiest ways to avoid spreading respiratory diseases such as colds and the flu. 

(Sources: http://www.cdc.gov/flu/images/CoverCgh-Sch-view.gif, http://www.cdc.gov/flu/images/CoverCgh- 
Sch-view.gif, License: Public Domain-gov, Minnesota Department of Health) 

Lesson Summary 

• Respiratory diseases are diseases that affect the lungs, bronchial tubes, trachea, nose, and throat. 
Respiratory diseases can reduce the amount of oxygen that gets into the blood. Asthma is an illness in 
which the bronchioles are inflamed and become narrow. The muscles around the bronchioles contract 
which narrows the airways further. 

• Difficulty in breathing happens because of the inflammation, contraction of the muscles, and the production 
of mucus by the cells that line the bronchioles. Diseases of the respiratory system that are caused by 
exposure to tobacco smoke include emphysema, lung cancer and cancers of the larynx and mouth. 

• Cigarette smoking causes 87 percent of lung cancer deaths. Smoking and using tobacco is also linked 
to the risk of developing other types of cancer. Avoiding smoking, getting enough exercise, and washing 
your hands often are three things you can do to help protect your respiratory system from illness. 

Further Reading I Supplemental Links 

http://www.cdc.gov/vaccines/vpd-vac/pertussis/default.htm 

http://www.cdc.gov/tobacco/data_statistics/fact_sheets/youth_data/youth_tobacco.htm 

http://www.cdc.gov/nceh/globalhealth/projects/airpollution.htm; http://www.cdc.gov/asthma/children.htm 

http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5644a2.htm; http://www.bmj.com/cgi/content/ab- 

stract/328/7455/1519 

http://www.cdc.gov/germstopper/home_work_school.htm; http://www.cdc.gov/flu/protect/keyfacts.htm 



494 



http://en.wikipedia.org/wiki/Cigarette_smoking; http://www.cancer.gov/cancertopics/factsheet/Tobacco/cancer 

http://www.cancer.gov/cancertopics/factsheet/Tobacco/cancer 

http://www.cdc.gov/tobacco/data_statistics/sgr/sgr_2004/sgranimation/html/welcome.html 

http://www.cdc.gov/flu/protect/covercough.htm 

Review Questions 

1 . Identify the organs that are affected by a respiratory disease. (Beginning) 

2. How might a respiratory disease affect the rest of the body? (Intermediate) 

3. How does asthma affects the bronchioles? (Intermediate) 

4. Medicines called bronchodilators are used to treat the symptoms of asthma. What action do you think 
these drugs have on the lungs? (Challenging) 

5. What lifestyle activity has the largest health impact on the respiratory system? (Beginning) 

6. Identify three diseases are linked to tobacco smoking. (Intermediate) 

7. Identify three things that can cause a respiratory disease. (Intermediate) 

8. What are two things you can do to keep your respiratory system healthy? (Beginning) 

9. Pneumonia is a disease in which the alveoli fill up with fluid. How might this affect the lungs' ability to 
absorb oxygen? (Challenging) 

10. How can washing your hands help prevent you from catching a cold? (Beginning) 
Vocabulary 

acute disease A disease that lasts a short time. 

allergen Any antigen that is not an infectious organism, such as mold, dust, or pet 

hair. 

asthma A chronic illness in which the bronchioles are inflamed and become nar- 

row. 

bronchodilators Drugs that reduce inflammation of the bronchioles allowing air through. 

bronchitis An inflammation of the bronchi. 

chronic bronchitis Having a cough that produces phlegm, for at least three months in a two- 

year period. 

chronic disease A disease that lasts for a long time, perhaps a few years or longer. 

chronic obstructive pulmonary A disease of the lungs in which the airways become narrowed; leads to 
disease (COPD) a limitation of the flow of air to and from the lungs causing shortness of 

breath. 

emphysema A chronic lung disease caused by loss of elasticity of the lung tissue. 

environmental tobacco smoke Secondhand smoke, which greatly increases the risk of lung cancer and 
(ETS) heart disease in nonsmokers. 

lifestyle disease A disease that is caused by choices that people make in their daily lives. 

lung cancer A disease where the cells that line the lungs grow out of control; the 

growing mass of cells pushes into nearby tissues and can affect how 
these tissues work. 



495 



pathogen An organism that causes disease in another organism; certain bacteria, 

viruses, and fungi are pathogens of the respiratory system. 

pertussis Whooping cough; gets its name from the loud whooping sound that is 

made when the person inhales during a coughing fit. 

pneumonia An illness in which the alveoli become inflamed and flooded with fluid. 

respiratory disease A disease of the lungs, bronchial tubes, trachea, nose, and/or throat. 

tuberculosis (TB) A common and often deadly infectious disease caused by a type of bac- 

terium called mycobacterium. 

Review Answers 

1. the organs of the respiratory system: the nasal passages, pharynx, larynx, trachea, and lungs 

2. a respiratory disease may lead to a reduction in the oxygen levels in the body 

3. during an asthma attack, the lining of the bronchioles become inflamed and the muscles around the 
bronchioles contract; cells that line the bronchioles produce a lot of mucus. These actions narrow the airways 
causing difficulty in breathing 

4. bronchodilators cause the bronchioles to dilate, allowing more air to enter the lungs 

5. smoking 

6. any three of the following: Cancer, heart disease, stroke, emphysema, COPD 

7. pollution, pathogens, lifestyle choices (smoking), heredity 

8. avoid smoking, wash my hands, stay active, get vaccinated 

9. oxygen cannot pass into the blood capillaries because air cannot get into the alveoli 

10. hand-washing removes cold viruses from the hands, or kill viruses that are on the hands 

Points to Consider 

• The respiratory system gets rids of a certain type of wastes. What type of wastes do you think are removed 
by your respiratory system? 

The Excretory System 

Lesson Objectives 

Identify the functions of the excretory system. 
List the organs of the excretory system. 
Describe the parts of urinary system. 
Outline how the kidneys filter blood. 
Identify three disorders of the urinary system. 

Check Your Understanding 

• What are some "wastes" that must be removed from your body? 



496 



• Do your circulatory and respiratory systems remove "waste?" 

Introduction 

One of the most important homeostatic jobs your body does it to keep the right amount of water and salts 
inside your body. Too much water and your cells would swell and burst. Too little water and your cells would 
shrivel up like an old apple. Either extreme would cause illness and death of cells, tissues, and organs. The 
organs of your excretory system help to keep the correct balance of water and salts within your body. 

Your body also needs to remove the wastes that build up from the metabolic activity of cells and digestion. 
These wastes include carbon dioxide, urea, and certain plant materials. If these wastes were not removed, 
your cells would stop working and you would get very sick. In this lesson you will learn how waste is removed 
from the body, and how the kidneys filter waste from the blood. 

The Excretory System 

The excretory system is the organ system that maintains homeostasis by keeping the correct balance of 
water and salts in your body. It also helps to release wastes from the body. Excretion is the process of re- 
moving wastes from the body. The organs of the excretory system are also parts of other organ systems. 
For example, your lungs are part of the respiratory system. Your lungs remove carbon dioxide from your 
body so they are also part of the excretory system. Table 1 lists more organs of the excretory system, and 
the other organs systems of which they are part. 



Table 1: Organs of the Excretory System 



Organ(s) 


Function 


Other Organ System of which it is 
Part 


Lungs 


Remove carbon dioxide 


Respiratory system 


Skin 


Sweat glands remove water, salts, and other wastes 


Integumentary system 


Large intes- 
tine 


Removes solid waste and some water in the form 
of feces 


Digestive system 


Kidneys 


Remove urea, salts, and excess water from the 
blood 


Urinary system 



Functions of the Excretory System 

The excretory system controls the chemical make-up of body fluids. The organs of the excretory system 
remove metabolic wastes. They also maintain the proper concentrations of water, salts, and nutrients in the 
body. In this way the excretory system has an important homeostatic job. 

Your body takes nutrients from food and uses them for energy, growth, and repair. After your body has taken 
what it needs from the food, waste products are left behind in the blood and in the large intestine. These 
waste products need to be removed from the body. The kidneys work with the lungs, skin, and intestines to 
keep the correct balance of nutrients, salts and water in your body. 

The Urinary System 

Sometimes and confusingly, the urinary system is called the excretory system. But, the urinary system is 
only a part of the excretory system. Recall that the excretory system is made up of the skin, lungs, and large 
intestine as well as the kidneys. The urinary system is the organ system that makes, stores, and gets rid 
of urine. It includes two kidneys, two ureters, the bladder, and the urethra. The urinary system is shown in 
Figure 1. 



497 



Components of the Urinary System 




Kidney 



reter 



Bladder 



rethra 



Figure 1: The kidneys filter the blood that passes through them and the urinary bladder stores the urine 
until it is released from the body. 

(Source: http://upload.wikimedia.0rg/wikipedia/commons/l/l8/lllu urinary system.jpg, License: Public Domain) 

Organs of the Urinary System 

As you can see from Figure 1, the kidneys are two bean-shaped organs. The kidneys filter and clean the 
blood and form urine. They are about the size of your fists and are found near the middle of the back, just 
below your rib cage. The ureters are tube-shaped structures that bring urine from the kidneys to the urinary 
bladder. The urinary bladder is a hollow, muscular, and elastic-walled organ. It is shaped a little like a 
balloon. It is the organ that collects urine which comes from the kidneys. Urine leaves the body through the 
urethra. 

What is Urine? 

Urine is a liquid that is formed by the kidneys when they filter wastes from the blood. Urine contains mostly 
water and also dissolved salts and nitrogen-containing molecules. The amount of urine excreted from the 
body depends on many things. Some of these include the amounts of fluid and food a person consumes 
and how much fluid they have lost in sweat and breathing. 

Urine is can range from colorless to dark yellow, but is usually a pale yellow color. Dilute urine is light yellow 
in color. Concentrated urine is dark yellow or may be brown. The darker the urine, the less water it contains. 

The urinary system removes a type of waste called urea from your blood. Urea is a nitrogen-containing 
molecule that is made when foods containing protein, such as meat, poultry, and certain vegetables, are 
broken down in the body. Urea and other wastes are carried in the bloodstream to the kidneys were they 
are removed and form urine. 

How the Kidneys Filter Wastes 

The kidneys are important organs in maintaining homeostasis. Kidneys perform a number of homeostatic 
functions: 

• Maintain the volume of body fluids 

• Maintain the balance of salt ions in body fluids 

• Excrete harmful nitrogenous wastes (metabolic by-products) such as urea, ammonia, and uric acid 



498 



There are many blood vessels in the kidneys, as you can see in Figure 2. The kidneys remove urea from 
the blood through tiny filtering units called nephrons. Nephrons are tiny, tube-shaped structures found inside 
each kidney. A nephron is shown in Figure 3. Each kidney has up to a million nephrons. Each nephron collects 
a small amount of fluid and waste products from a small group of capillaries. If the body is in need of more 
water, water is removed from the fluid inside the nephron and is returned to the blood. The fluid within 
nephrons is carried out into a larger tube in the kidney called a ureterwhich you can see in Figure 2. Urea, 
together with water and other wastes, forms the urine as it passes through the nephrons and the kidney. 




Figure 2: Structures of the kidney; fluid leaks from the capillaries and into the nephrons where the fluid 
forms urine then moves to the ureter and on to the bladder. 

(Source: http://commons.wikimedia.Org/wiki/lmage:Kidney PioM.png, Image by: Piotr Michat Jaworski, License: 
GFDL1.2) 





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Figure 3: The location of nephrons in the kidney; the glomerulus is the network of blood vessels that filter 
liquid into the nephron, collects in the nephron tubules, and moves to the bladder through the ureter. 



499 



(Source: http://kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/, License: NC-Gov) 
Formation of Urine 

The process of urine formation is as follows: 

1. Blood flows into the kidney through the renal artery, shown in Figure 2. The renal artery branches into 
capillaries inside the kidney. Capillaries and the nephrons lie very close to each other in the kidney. 

2. The blood pressure within the capillaries causes water and solutes such as salts, sugars, and urea to 
leave the capillaries and move into the nephron. 

3. The water and solutes move along through the tube-shaped nephron to a lower part of the nephron. At 
this point most of the water and solutes are returned to the capillaries that surround the nephron. 

4. The fluid that remains in the nephron at this point is called urine. 

5. The blood that leaves the kidney in the renal vein has much less waste than the blood that entered the 
kidney. 

6. The urine is collected in the ureters and is moved to the urinary bladder where it is stored. 

Nephrons filter 125 ml (about % cup) of body fluid per minute. In a 24-hour period nephrons produce about 
1 80 liters of filtrate, of which 1 78.5 liters are reabsorbed. The remaining 1 .5 liters of fluid forms urine. 

Urine enters the bladder through the ureters. Similar to a balloon, the walls of the bladder are stretchy. The 
stretchy walls allow the bladder to hold a large amount of urine. The bladder can hold about 400 to 620 ml_ 
(about V/ 2 to 214 cups) of urine, but may also hold more if the urine cannot be released immediately. Urination 
is the process of releasing urine from the body. Urine leaves the body through the urethra. 

Nerves in the bladder tell you when it is time to urinate. As the bladder first fills with urine, you may notice 
a feeling that you need to urinate. The urge to urinate becomes stronger as the bladder continues to fill up. 

Brain Control 

The kidneys never stop filtering waste products from the blood, so they are always producing urine. The 
amount of urine your kidneys produce is dependent on the amount of fluid in your body. Your body loses 
water through sweating, breathing, and urination. The water and other fluids you drink every day help to 
replace the lost water. This water ends up circulating in the blood because blood plasma is mostly water. 

The kidneys will normally adjust to the level of water a person drinks. For example, if a person suddenly in- 
creases their water intake, the kidneys will produce more diluted (watery) urine. If a person drinks much 
less fluid than they usually do, their urine will be more concentrated (contain much less water). 

The filtering action of the kidneys is controlled by the pituitary gland. The pituitary gland is about the size of 
a pea and is found below the brain, as shown in Figure 4. The pituitary gland is also part of the endocrine 
system. The pituitary gland releases hormones which affect the ability of the kidneys to filter water from the 
blood. 

The absorption of water back into blood is controlled by a hormone called antidiuretic hormone (ADH). ADH 
is released from the pituitary gland in the brain. One of the most important roles of ADH is to control the 
body's ability to hold onto water. If a person does not drink enough water, ADH is released and it causes 
the kidneys to remove more water from the urine. The urine is more concentrated and is less in volume. 

When too much fluid is present in the blood, the amount of ADH in the blood is reduced. This increases the 
amount of water that filters into the nephrons. The kidneys then produce a large volume of more dilute urine. 



500 




Figure 4: The pituitary gland is found directly below the brain and releases hormones that control the pro- 
duction of urine. 

(Source: http://www.flickr.com/photos/jcrojas/523383835/, Photographer: J.C. Rojas, License: CC-BY-SA 
2.5) 

Excretory System Problems 

The urinary system controls the amount of water in the body, and removes wastes, so any problem with the 
urinary system can badly affect many other body systems. Some common urinary system problems are 
described here. 

Kidney Stones 

In some cases, certain mineral wastes in urine crystallize and form kidney stones like the one shown in 
Figure 5. Stones form in the kidneys and may be found anywhere in the urinary system. They vary in size. 
Some stones cause great pain while others cause very little pain. Some stones may need to be removed 
by surgery or ultrasound treatments. 




501 



Figure 5: A kidney stone; the stones can form anywhere in the urinary system. 

(Source: http://en.wikipedia.0rg/wiki/lmage:Nefr0lit.jpg, Image by: Robert R. Wal, License: Public Domain) 

Kidney failure 

Kidney failure results when the kidneys are not able to regulate water and chemicals in the body or remove 
waste products from the blood. If the kidneys are unable to filter wastes from the blood, the wastes build up 
in the body. Homeostasis is disrupted because the ions and fluids in the body are out of balance. 

Kidney failure can be caused by an accident that injures the kidneys, the loss of a lot of blood, or it can be 
caused by some drugs or poisons. Kidney failure may lead to permanent loss of kidney function. But if the 
kidneys are not seriously damaged, they may recover. Chronic kidney disease is the gradual reduction of 
kidney function that may lead to permanent kidney failure. 

A person who has lost kidney function may need to undergo kidney dialysis. Kidney dialysis is the process 
of artificially filtering the blood of wastes. A dialysis machine (also called a hemodialyzer) filters the blood 
of waste by pumping it through a semipermeable membrane. The cleansed blood is then returned to the 
patient's body. A dialysis machine is shown in Figure 6. 



Hemodialyzer 
j/ (Where filtering takes place) 




Hemodialysis Unfiltered blood Filtered blood 
machine flows to dialyzer flows Back 10 Dody 



Figure 6: During hemodialysis, a patient's blood is sent through a filter that removes waste products and 
the clean blood is returned to the body. 

(Source: http://kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys/, License: NC-Gov) 

Urinary tract infections (UTIs) 

Urinary tract infections are bacterial infections of any part of the urinary tract. When bacteria get into the 
bladder or kidney and multiply in the urine, they cause a UTI. The most common type of UTI is a bladder 
infection. Women get UTIs more often than men. UTIs are often treated with antibiotics. 

Lesson Summary 

• The excretory system controls the chemical make-up of body fluids. The organs of the excretory system 
remove metabolic wastes. They also maintain the proper concentrations of water, salts, and nutrients in 
the body. 



502 



• The lungs, skin, kidneys, and large intestine are all part of the excretory system. The urinary system is 
made up of the kidneys, the ureters, the bladder, and the urethra. The filtering structures of the kidneys 
are the nephrons. 

• Water and waste molecules move out of the blood capillaries and into the nephrons. Most of the water 
returns to the blood. Urine collects in the nephron and moves to the urinary bladder through the ureters. 

• The filtering action of the kidneys is controlled by the pituitary gland. ADH is the hormone that controls 
the uptake of water from the kidneys. Disorders of the urinary system include kidney stones, kidney 
disease and urinary tract infections. 

Review Questions 

1 . What are the functions of the excretory system? (Beginning) 

2. List the organs that make up the excretory system. (Beginning) 

3. What is the difference between the urinary system and the excretory system? (Intermediate) 

4. What is urine made up of? (Beginning) 

5. Outline how the kidneys filter blood. (Intermediate) 

6. What is the purpose of the urinary bladder? (Beginning) 

7. The walls of the urinary bladder are stretchy, what do you think is the advantage to having these stretchy 
walls? (Intermediate) 

8. What connects the kidneys to the urinary bladder? (Beginning) 

9. What does antidiuretic hormone do? (Intermediate) 

10. What is a urinary tract infection? (Intermediate) 

11. Why is kidney failure such a serious health problem? (Intermediate) 
Further Reading I Supplemental Links 
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEXCRETE 
http://kidney.niddk.nih.gov/kudiseases/pubs/yourkidneys 
http://en.wikipedia.org/wiki 

Vocabulary 

antidiuretic hormone (ADH) Hormone that controls the absorption of water back into blood. 

excretion The process of removing wastes from the body. 

excretory system The organ system that maintains homeostasis by keeping the correct balance 

of water and salts in your body; also helps to release wastes from the body. 

homeostasis The ability to maintain a stable internal environment despite external changes. 

kidney Organ that filters and cleans the blood and forms urine; also maintains the 

volume of body fluids, maintains the balance of salt ions in body fluids, and 
excretes harmful metabolic by-products such as urea, ammonia, and uric 
acid. 

kidney dialysis The process of artificially filtering the blood of wastes; a patient's blood is 

sent through a filter that removes waste products and the clean blood is re- 



503 



turned to the body. 

kidney failure When the kidneys are not able to regulate water and chemicals in the body 

or remove waste products from the blood. 

kidney stone "Stones" formed when certain mineral wastes in urine crystallize; may be 

found anywhere in the urinary system. 

nephron Tiny, tube-shaped filtering unit found inside each kidney. 

urea A nitrogen-containing molecule that is made when foods containing protein, 

such as meat, poultry, and certain vegetables, are broken down in the body. 

ureter Tube-shaped structure that brings urine from the kidneys to the urinary 

bladder. 

urethra Structure through urine leaves the body. 

urinary bladder Organ that collects the urine which comes from the kidneys. 

urinary system The organ system that makes, stores, and gets rid of urine. 

urinary tract infection (UTI) Bacterial infections of any part of the urinary tract. 

urination The process of releasing urine from the body. 

urine A liquid that is formed by the kidneys when they filter wastes from the blood; 

contains mostly water and also dissolved salts and nitrogen-containing 
molecules. 

Review Answers 

1 . the organs of the excretory system remove metabolic wastes, and maintain the proper concentrations of 
water, salts, and nutrients in the body 

2. the skin, lungs, kidneys, and large intestine 

3. the urinary system includes the kidneys, ureters, bladder, and urethra; the excretory system includes the 
kidneys, lungs, skin, and large intestine 

4. urine contains mostly water and also dissolved salts and nitrogen-containing molecules 

5. the kidneys remove urea from the blood through the nephrons; each nephron collects a small amount of 
fluid and waste products from a small group of capillaries and the fluid within nephrons is carried out into 
the ureters 

6. the urinary bladder stores urine until it can be released from the body 

7. the stretchy walls allow the bladder to expand to hold more urine 

8. the ureters connect the kidneys to the bladder 

9. it controls the reabsorption of water from the urine and into the blood 

1 0. a urinary tract infection is a bacterial infection of any part of the urinary tract 

11. when the kidneys cannot filter wastes from the blood, the wastes build up in the blood and body and 
affect homeostasis 

Points to Consider 

• Next we turn our attention to the nervous system. What do you think the nervous system is? What do 
you think it does? 



504 



20. Controlling the Body 



The Nervous System 

Lesson Objectives 

• Identify the functions of the nervous system. 

• Describe neurons and explain how they carry nerve impulses. 

• Describe the structures of the central nervous system. 

• Outline the divisions of the peripheral nervous system. 

Check Your Understanding 

• If groups of cells are called tissues and groups of tissues are called organs, what are groups of organs 
called? 

• What are examples of human organ systems? 

• Which organ system controls all the others? 

Introduction 

Groups of organs called organ systems. Examples of human organ systems are skeletal, digestive, and 
respiratory systems. The nervous system controls all the others. 

Michael was riding his scooter when he hit a hole in the sidewalk and started to lose control. He thought he 
would fall, but in the blink of an eye, he shifted his weight and regained his balance. His heart was pounding, 
but at least he didn't get hurt. How was he able to react so quickly? Michael can thank his nervous system 
for that. 




Figure 1: Staying balanced when riding a scooter requires control over the body's muscles; the nervous 
system controls the muscles and maintains balance. 



505 



(Source: http://us.fotolia.com/id/631633 Tomasz Trojanowski, License: Royalty Free) 
What Does the Nervous System Do? 

The nervous system is the body system that controls all the other systems of the body. Controlling muscles 
and maintaining balance are just two of its roles. The nervous system also lets you: 

Senses your surroundings with your eyes and other sense organs. 
Senses your internal environment, including temperature and pH. 
Controls your internal body systems and keeps them in balance. 
Prepares your body to fight or flee in emergency situations. 
Thinks, learns, remembers, and uses language. 

The nervous system works by sending and receiving electrical messages. The messages are carried by 
nerves throughout the body. For example, when Michael started to fall off his scooter, his nervous system 
sensed that he was losing his balance. It responded by sending messages to muscles throughout his body. 
Some muscles tightened while others relaxed. As a result, Michael's body became balanced again. How 
did his nervous system do all that in just a split second? To answer this question, you need to know how 
the nervous system carries messages. 

Neurons and Nerve Impulses 

The nervous system is made up of nerves. A nerve is a bundle of individual nerve cells. A nerve cell that 
carries messages is called a neuron (Figure 2). The messages carried by neurons are referred to as nerve 
impulses. Nerve impulses are able to travel very quickly because they are electrical impulses. Think about 
flipping on a light switch when you enter a room. When you flip the switch, it closes an electrical circuit. With 
the circuit closed, electricity can flow to the light through wires inside the walls. The electricity may have to 
travel many meters to reach the light, but the light still comes on as soon as you flip the switch. Nerve impulses 
travel equally fast through the network of nerves inside the body. 



Structure of a Typical Neuron 



Dendrite 



Nucleus 




Axon terminal 



Schwann cell 
Myelin sheath 



Figure 2: The axons of many neurons, like the one shown here, are covered with a fatty layer called myelin 
sheath that insulates the axon like the plastic covering on an electrical wire, and allowing nerve impulses to 
travel faster along the axon. 

(Source: http://c0mm0ns.wikimedia.0rg/wiki/lmage:Neur0n.jpg, License: Public Domain) 



506 



What Does a Neuron Look Like? 

A neuron has a special shape that lets it pass signals from one cell to another. As shown in Figure 2, a 
neuron has three main parts: cell body, dendrites, and axons. The cell body contains the nucleus and other 
organelles. Dendrites and axons project from the cell body. Dendrites receive nerve impulses from other 
cells, and axons pass the nerve impulses on to other cells. A single neuron may have thousands of dendrites 
and axons, so it can communicate with thousands of other cells. 

Types of Neurons 

Neurons are usually classified based on the role they play in the body. Two types of neurons are sensory 
neurons and motor neurons. 

• Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous 
system (see below). 

• Motor neurons carry nerve impulses from the central nervous system to internal organs, glands, and 
muscles. 

Both types of neurons work together. Sensory neurons carry information about conditions inside or outside 
the body to the central nervous system. The central nervous system processes the information and sends 
message through motor neurons telling the body how to respond to the information. 

The Synapse 

The place where the axon of one neuron meets the dendrite of another is called a synapse. Synapses are 
also found between neurons and other type of cells, such as muscle cells. The axon of the sending neuron 
doesn't actually touch the dendrite of the receiving neuron. There is a tiny gap between them, as shown in 
Figure 3. 



neurotransmetteurs 




►• j-syn 



apse 



Figure 3: This diagram shows a synapse between neurons; when a nerve impulse arrives at the tip of the 
axon, neurotransmitters are released and travel to the receiving dendrite, carrying the nerve impulse from 
one neuron to the next. 

(Source: http://commons.wikimedia.org/wiki/lmage: Synapse. png, License: Public Domain) 

When a nerve impulse reaches the tip of an axon, the axon releases chemicals called neurotransmitters. 
These chemicals travel across the gap between the axon and the dendrite of the next neuron. They bind to 
the membrane of the dendrite. This triggers a nerve impulse in the receiving neuron. Did you ever watch a 
relay race? After the first runner races, she passes the baton to the next runner, who takes over. Neurons 
are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron. Examples 
of neurotransmitters include serotonin, dopamine, and adrenaline. 



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You can watch an animation of nerve impulses and neurotransmitters at: http://www.mind.ilstu.edu/curricu- 
lum/neurons_intro/neurons_intro.php 

Some people have low levels of the neurotransmitter serotonin in their brain. Scientists think that this is 
one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. 
For many people with depression, antidepressants control the symptoms of their depression and help them 
lead happy, productive lives. 

Central Nervous System 

The central nervous system (CNS) is the largest part of the nervous system. As shown in Figure 4, it in- 
cludes the brain and the spinal cord. The brain is protected within the bony skull. The spinal cord is protected 
within the bones of the spine, which are called vertebrae. 



Bfain — 



Cerlrel nervous 
iystem (brain 
and spiral cord) 




Spinal «xd 



Figure 4: The brain and spinal cord make up the central nervous system. 

(Source: http://commons.wikimedia.org/wiki/lmage: Central nervous system.gif, License: GNU Free Docu- 
mentation) 

The Brain 

What weighs about 3 pounds (1 .5 kilograms) and contains up to 1 00 billion cells? The answer is the human 
brain. The brain is the control center of the nervous system. It's like the pilot of a plane. It tells other parts 
of the nervous system what to do. The brain is also the most complex organ in the body. Each of its 100 
billion neurons has synapses connecting it with thousands of other neurons. All those neurons use a lot of 
energy. In fact, the adult brain uses almost a quarter of the total energy used by the body. The developing 
brain of a baby uses an even greater percentage of the body's total energy. 

The brain is the organ that lets us interpret what we see, hear, or sense in other ways. It also allows us to 
learn, think, remember, and use language. The brain controls all of our internal body processes and external 
movements, as well. As shown in Figure 5a, the brain consists of three main parts: 

• The cerebrum is the largest part of the brain. It lies on top of the brainstem (discussed below). The 
cerebrum controls functions that we are aware of, such as problem-solving and speech. It also controls 
voluntary movements, like waving to a friend. Whether you are doing your homework or jumping hurdles, 
you are using your cerebrum. 

• The cerebellum is the next largest part of the brain. It lies under the cerebrum and behind the brain 
stem. The cerebellum controls body position, coordination, and balance. Whether you are riding a bicycle 
or writing with a pen, you are using your cerebellum. 

• The brain stem is the smallest of the three main parts of the brain. It lies directly under the cerebrum. 
The brain stem controls basic body functions such as breathing, heartbeat, and digestion. The brain 



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stem also carries information back and forth between the cerebrum and spinal cord. 





Figure 5b: Top view of the brain and cerebrum; 
Figure 5a: Side view of the brain; find the location of the divided from front to back into two halves, these 
three major parts of the brain, noting that the cerebrum are the right and left hemispheres, 
is divided into four lobes at the upper portion of the brain: 
the frontal, parietal, temporal, and occipital lobes. 



(Sources: http://upload.wikimedia.Org/wikipedia/commons/9/90/Brainlobes.png, http://commons.wikime- 
dia.org/wiki/lmage: Hemispheres. png, Licenses: Public Domain, GNU Free Documentation) 

The cerebrum is divided into a right and left half, as shown in Figure 5b. Each half of the cerebrum is called 
a hemisphere. The two hemispheres are connected by a thick bundle of axons called the corpus callosum. 
It lies deep inside the brain and carries messages back and forth between the two hemispheres. The right 
hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body. 
This would be impossible without the corpus callosum. 

Dr. Jill Bolte Taylor is a brain scientist. At the age of 37, she suffered massive brain damage when blood 
vessels burst inside her brain. It took Dr. Bolte almost ten years to recover from the damage to her brain. 
She had to relearn even basic skills, like walking and talking. To share her story of recovery with others, Dr. 
Taylor wrote a popular book describing what she went through. Her story gave other people so much inspi- 
ration that Time Magazine named her one of the world's 100 most influential people in 2008. 

Each hemisphere of the cerebrum is divided into four parts called lobes. The four lobes are the frontal, 
parietal, temporal, and occipital lobes (see Figure 5a). Each lobe has different functions. Some of the functions 
are listed in Table 1. 



Cerebral Lobes and Their Functions 



Lobe 


Main Function(s) 


Frontal 


Speech, thinking, touch 


Parietal 


Speech, taste, reading 


Tempo- 
ral 


Hearing, smell 


Occipital 


Sight 



Table 1: Cerebrum Lobe Functions 

(Created by: Jean Brainard) 

The Spinal Cord 

The spinal cord is a long, tube-shaped bundle of neurons. It runs from the brain stem to the lower back. 
The main job of the spinal cord is to carry nerve impulses back and forth between the body and brain. The 



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spinal cord is like a two-way highway. Messages about the body, both inside and out, pass through the 
spinal cord to the brain. Messages from